Mechanisms for utilizing a model space trim curve to provide inter-surface continuity

ABSTRACT

A mechanism is disclosed for reconstructing trimmed surfaces whose underlying spline surfaces intersect in model space, so that the reconstructed version of each original trimmed surface is geometrically close to the original trimmed surface, and so that the boundary of each respective reconstructed version includes a model space trim curve that approximates the geometric intersection of the underlying spline surfaces. Thus, the reconstructed versions will meet in a continuous fashion along the model space curve. The mechanism may operate on already trimmed surfaces such as may be available in a boundary representation object model, or, on spline surfaces that are to be trimmed, e.g., as part of a Boolean operation in a computer-aided design system. The ability to create objects with surface-surface intersections that are free of gaps liberates a whole host of downstream industries to perform their respective applications without the burdensome labor of gap repair, and thus, multiplies the efficacy of those industries.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.15/433,823, filed on Feb. 15, 2017, entitled “Mechanisms forConstructing Spline Surfaces to Provide Inter-Surface Continuity”, byBenjamin Urick et al., which claims benefit of priority to ApplicationNo. 62/295,892 titled “Construction of Spline Surfaces to ProvideInter-Surface Continuity”, filed on Feb. 16, 2016, and Application No.62/417,781 titled “Mechanisms for Constructing Spline Surfaces toProvide Inter-Surface Continuity”, filed on Nov. 4, 2016, and which areall hereby incorporated by reference as though fully and completely setforth herein.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Grant No.N00014-08-1-0992 awarded by the Office of Naval Research. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to the field of computer-aideddesign (CAD), and more particularly, to mechanisms for constructing(including reconstructing) tensor product spline surfaces to address thetrim problem.

DESCRIPTION OF THE RELATED ART

The computer-aided design of models for objects is fundamentallyimportant to a variety of industries such as computer-aided engineering(CAE), computer-aided manufacturing (CAM), computer graphics andanimation. For example, a model may be used as the basis of anengineering analysis, to predict the physical behavior of the object.The results of such analysis may be used in a wide variety of ways,e.g., to inform changes to the design of the object, to guide selectionof material(s) for realization of the object, to determine performancelimits (such as limits on temperature, vibration, pressure, shearstrength, etc.), and so forth. As another example, a model may be usedto direct the automated manufacturing of the object. As yet anotherexample, a model may be used to generate an image (or a sequence ofimages, e.g., as part of an animation of the object). These activitiesmay represent typical product design development steps found acrossindustrial market verticals such as automotive, aerospace, and oil andgas.

Modern premiere CAD software applications may be built on softwarekernels that utilize restrictive mathematical assumptions to approximatecompound geometric objects. As a result, critical information may not beexplicitly modeled, forcing designers, engineers, manufacturers,animators, etc., to repair CAD models and convert them into anacceptable format, such as polygonal meshes for Finite Element Analysis(FEA) or Computational Fluid Dynamics (CFD), 3-D printing or additivemanufacturing, character animation, etc. This conversion is typically aniterative process, creating substantial amounts of work for productdevelopment teams. Designers, engineers, manufacturers, animators, etc.,waste countless hours manually repairing gaps in models and dealing withredundant one-way file conversion operations, causing significantproductivity losses, increased time to market and user frustration anddissatisfaction. As such, improvements in the field of CAD modeling maybe desirable.

SUMMARY

Embodiments are presented herein of methods, computer systems, andcomputer-readable memory media for constructing gapless surface modelsin computer-aided design (CAD) applications. In one set of embodiments,a computer-implemented method for modifying a model is implemented,wherein the method comprises performing, by the computer: storinggeometric input data describing a first and second input parametricsurface associated with the model, wherein the first and second inputparametric surfaces are described in a first and second parameter spacedomain, respectively. The method may proceed by storing a model spacetrim curve associated with the first and second input surfaces, whereinthe model space trim curve approximates a geometric intersection of thefirst and second input surfaces. The method may proceed byreparametrizing the first and second parametric surfaces into a commonthird parameter space domain based on the model space trim curve. Themethod may proceed by constructing first and second output surfaces,wherein the first and second output surfaces are described in the thirdparameter space domain, wherein at least a portion of the boundary ofeach of the first and second output surfaces coincides with the modelspace trim curve. Finally, the method may output the first and secondoutput surfaces as modified components of the model.

In some embodiments, the first input surface may include a first tensorproduct spline surface; the first geometric input data may include afirst pair of knot vectors and a first set of surface control points andweights; the second input surface may include a second tensor productspline surface; and the second geometric input data may include a secondpair of knot vectors and a second set of surface control points andweights.

In some embodiments, the first and second tensor product spline surfacesmay be NURBS surfaces (or T-Spline surfaces).

Some embodiments may comprise a pre-SSI algorithm, wherein it is assumedthe user has two boundary representations (Brep, brep, B-rep or b-rep)they wish to perform a solid modeling Boolean operation on. Otherembodiments may comprise a post-SSI algorithm, wherein it is assumed theuser has a valid B-rep model that contains the results of previouslycomputed, standard CAD solid modeling Boolean operations. Pre-SSI inputdata may comprise two B-reps upon which it is desired to perform solidmodeling Boolean operations. Post-SSI input data may comprise a validB-rep model that contains the results of previously computed, standardCAD solid modeling Boolean operations. For both the pre-SSI and post-SSIalgorithms, geometric input data may be stored describing a first andsecond input surface associated with the model under consideration. Eachof the first and second input surfaces may be described as parametricsurfaces in a first and second respective parameter space domain.

The input for the Post-SSI algorithm may be a boundary representationcreated using one or more (typically many) solid modeling Booleanoperations on spline surfaces. (The spline surfaces may include tensorproduct spline surfaces such as NURBS surfaces and/or T-Splinesurfaces.) In one embodiment, the boundary representation may beprovided in a computer-aided design (CAD) file. In another embodiment,the Post-SSI algorithm may operate as part of the software kernel of aCAD system, in which case the boundary representation may have aspecialized internal format.

In some embodiments, the first output surface geometrically approximatesa subsurface of the first input surface to within model tolerance, andthe second output surface geometrically approximates a subsurface of thesecond input surface to within the model tolerance, wherein modeltolerance may be predetermined or user-defined.

In some embodiments, the boundary portion of the first output surfacepatch corresponds to a boundary isoparametric curve of the first outputsurface patch, wherein the boundary portion of the second output surfacecorresponds to a boundary isocurve of the second output surface. (Notethat the term “isocurve” is used as a synonym for isoparametric curve.)

In some embodiments, the method may also include: numerically computinga set of intersection points that at least approximately reside on thegeometric intersection of the first input surface and the second inputsurface; computing geometric data (e.g., knot vector and curve controlpoints) that specify a model space curve, approximating the intersectionof the first and second input surfaces as a parametric curve, based onthe set of intersection points; and storing the geometric data thatspecify the model space curve.

In some embodiments, the method may also include executing anengineering analysis (e.g., a physics-based analysis) based on themodified model, to obtain data predicting physical behavior of theobject. (The predictive data may be used to: generate display output toa user, for visualization of the physical behavior; calculate and outputa set of performance limits for a manufactured realization of theobject; identify locations of likely fault(s) in a manufacturedrealization of the object; select material(s) to be used for manufactureof the object; direct a process of manufacturing the object; directautomatic changes to geometry of the boundary representation; etc.)

In some embodiments, the above mentioned engineering analysis maycomprise an isogeometric analysis.

In some embodiments, the method may also include: after having performedsaid modification of the model, manufacturing (or directing a process ofmanufacturing) the object based on the boundary representation.

In some embodiments, the method may also include: after having performedsaid modification of the model, generating an image of the object basedon the boundary representation and displaying the image.

In some embodiments, the method may also include: after having performedsaid modification of the model, generating a sequence of animationimages based on the boundary representation and displaying the sequenceof animation images.

In some embodiments, the action of creating the first geometric outputdata includes: determining a first set of isocurve data specifying afirst set of isocurves of the first input surface on a region within aparametric domain of the first input surface, dividing the first set ofisocurves at respective locations based on the model space curve, toobtain a second set of isocurve data specifying sub-isocurves of therespective isocurves; and computing surface control points for the firstoutput surface patch based on control points for said portion of themodel space curve and a proper subset of the second set of isocurvedata. (In this context, “proper subset” means a non-empty subset thatexcludes the end control points nearest the model space curve.)

In some embodiments, the above described sub-isocurves arereparametrized to a common parametric interval and cross-refined toachieve a common knot vector prior to said computing the surface controlpoints for the first output surface patch.

In some embodiments, the action of computing the surface control pointsfor the first output surface patch comprises solving one or more linearsystems of equations that relate said surface control points to thecontrol points for said portion of the model space curve and said atleast a portion of the first set of isocurve data.

In some embodiments, the method may operate in the context of surfacemodeling, in which a topological object model (i.e., faces, edges, andvertices) has been created to reference the surface model objects (i.e.,the tensor product spline surfaces, the model space curve, and aparameter space curve per tensor product spline).

Thus, the two output surfaces may meet in a C⁰ continuous fashion alongthe model space trim curve, corresponding to the isoparametric curveboundaries of the surfaces, and closely respect the CAD designer'sselection of original geometry for the trimmed surfaces. (The modelspace trim curve is associated with the topological edge between the twotrimmed surfaces.) Thus, after application of this method, theabove-described gaps at surface-surface intersections may no longerexist as per the user's intent. Having made the surfaces watertight, theboundary representation may be immediately ready for any of variousapplications such as finite element analysis (e.g., conventionalanalysis or isogeometric analysis), graphics rendering, animation, andmanufacturing (e.g., 3D printing, such as additive manufacturing). Forexample, conventional mechanisms may be invoked to automatically convertthe boundary representation into a continuous polygonal mesh, for any ofthe above listed downstream processes. As another example, the boundaryrepresentation may be subjected to an isogeometric analysis, in whichthe geometric basis functions, implicit in the output surfaces orrefinements thereof, are used as the analysis basis functions.

Note that each of the above described output surfaces may be representedby a corresponding set of one or more output surface patches. Thus, theabove stated conditions on the two output surfaces may be interpreted asconditions on the two sets of output patches. In particular, for eachset of output patches, the boundary of the union of the images of theset of output patches includes the model space trim curve, and the unionof the images of the set of output patches is geometrically close to thecorresponding trimmed surface.

Furthermore, the above described method naturally generalizes to thereconstruction of two or more trimmed surfaces that nominally intersectalong a given model space trim curve. (We use the term “nominally”because the trimmed surfaces actually meet in a non-continuousgap-replete fashion.) Thus, in some embodiments, the method may operateon the two or more trimmed surfaces to generate two or morecorresponding output surfaces that meet in a C⁰ continuous fashion alongthe model space trim curve, and closely respect the CAD designer'sselection of original geometry for the trimmed surfaces. Theseembodiments may be especially useful for the design of non-manifoldobjects, or objects including non-manifold structures. (As an example ofhow more than two nominally intersecting trimmed surfaces might arise,imagine the Boolean union of two arbitrary spline surfaces thatintersect in model space. This operation would give four trimmedsurfaces that nominally intersect along a model space trim curve.)

In prior art solid modeling technology, the geometry of the splinesurfaces S₀ and S₁ is never altered when performing the Booleanoperations on the spline surfaces. Unfortunately, this commitment tounaltered geometry makes it impossible for the conventional Booleanoperations to create surface-surface intersections that are gap free.(Typically, surface-surface intersections exhibit numerous small scalegaps and openings, making the model non-watertight.) The Brep objectresulting from a conventional Boolean operation is un-editable andstatic. As a result, downstream processes such as finite elementanalysis and 3D printing cannot be performed until the solid model isrebuilt, typically with many hours of painstaking human labor.

According to embodiments presented herein, continuity of surface-surfaceinterface may ensure that the boundary representation model isimmediately ready for downstream applications such as engineeringanalysis or manufacture or graphics rendering. Because there are no gapsat the surface-surface interfaces, there may be no need for gapremediation.

In some embodiments, the action of constructing the boundaryrepresentation model of the object may be performed as part of a solidmodeling Boolean operation in a CAD software system on a set of two ormore input surfaces, wherein each of the output surfaces corresponds toa respective one of the input surfaces. In some embodiments, the actionof constructing the boundary representation may be performed internal toa CAD software application.

In some embodiments, for each pair of the input surfaces thatgeometrically intersect, the corresponding pair of output surfaces meetin a continuous fashion along respective isocurves.

The method for creating the output patches may do so in a watertightconfiguration, such that the output surfaces meet in at least a C⁰continuous fashion along respective isocurves. This method may providegeometric as well as parametric compatibility, in which the domains ofthe output surfaces are created to define a previously missing singledomain for the output surfaces for which desirable properties arefurnished (e.g., properties such as geometric and parametric continuity,parametric structure, parametric fidelity, mesh resolution, mesh aspectratio, mesh skew, mesh taper, etc.).

In some embodiments, the method may also include: (a) performing acomputer-based engineering analysis on the object based on the boundaryrepresentation model (without any need for gap-remediation on theboundary representation model), wherein the computer-based engineeringanalysis calculates data representing physical behavior of the object;and (b) storing and/or displaying the data representing the physicalbehavior of the object. In some embodiments, the data represents one ormore of: a predicted location of a fault in the object; a decision onwhether the object will tolerate (or survive or endure) a user specifiedprofile of applied force (or pressure or stress or thermal stimulus orradiation stimulus or electromagnetic stimulus); a predicted thermalprofile (and/or stress profile) of an engine (or nuclear reactor) underoperating conditions.

In some embodiments, the method may also include directing one or morenumerically controlled machines to manufacture the object based on theboundary representation model (without any need for gap-remediation onthe boundary representation model). The physical surfaces of themanufactured object meet in a continuous fashion. In some embodiments,the numerically controlled machines include one or more of thefollowing: a numerically controlled mill, a numerically controlledlathe, a numerically controlled plasma cutter, a numerically controlledelectric discharge machine, a numerically controlled fluid jet cutter, anumerically controlled drill, a numerically controlled router, anumerically controlled bending machine.

In some embodiments, the method may also include: (a) employing a 3Dgraphics rendering engine to generate a 3D graphical model of the objectbased on the boundary representation model (without any need for gapremediation on the boundary representation model); and (b) storingand/or displaying the 3D graphical model of the object (e.g., as part ofa 3D animation or movie).

In some embodiments, the method may also include: (a) performing 3D scanon the boundary representation model to convert the boundaryrepresentation model into a data file for output to a 3D printer(without any need for gap remediation on the boundary representationmodel); and (b) transferring the data file to a 3D printer in order toprint a 3D physical realization of the object. Due to the continuity ofmeeting between surfaces of the boundary representation, the printedphysical realization will have structural integrity (and not be subjectto falling apart at surface-surface interfaces).

In some embodiments, the method may also include manufacturing a portionof a body (e.g., a hood or side panel or roof section) of an automobilebased on the boundary representation model of the object. Surfaces ofthe body portion meet in a continuous fashion.

In some embodiments, the method may also include manufacturing a portionof a body of a boat (or submarine) based on the boundary representationmodel of the object. Surfaces of the body portion meet in a continuousfashion.

In some embodiments, the method may also include: (a) downloading eachof the output surfaces to a corresponding robotic manufacturing device;and (b) directing the robotic manufacturing devices to manufacture therespective output surfaces. Furthermore, the method may also includeassembling the manufactured output surfaces to form a composite physicalobject.

In some embodiments, the boundary representation model represents ahydrocarbon reservoir in the earth's subsurface. In these embodiments,the method may include performing a geophysics simulation based on theboundary representation model in order to predict physical behavior(e.g., a flow field or a pressure field or a temperature-pressure field)of one or more hydrocarbons in the hydrocarbon reservoir. The method mayalso include determining one or more geographic locations for drillingof one or more exploration and/or production wells in the hydrocarbonreservoir. The method may also include determining a time profile forproduction of the one or more hydrocarbons via one or more wells basedon the predicted physical behavior.

This summary is intended to provide a brief overview of some of thesubject matter described in this document. Accordingly, it will beappreciated that the above-described features are merely examples andshould not be construed to narrow the scope or spirit of the subjectmatter described herein in any way. Other features, aspects, andadvantages of the subject matter described herein will become apparentfrom the following Detailed Description, Figures, and Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theattached drawings in which:

FIG. 1A is an illustration of intersecting first and second splinesurfaces, according to some embodiments;

FIG. 1B is an illustration of the intersecting first and second splinesurfaces of FIG. 1A that have been trimmed along their intersection,according to some embodiments;

FIG. 1C is an illustration of the gaps that may occur along the trimmedintersection of FIG. 1B, according to some embodiments;

FIG. 2 is an illustration of an exemplary computer system that may beused to perform any of the method embodiments described herein,according to some embodiments;

FIG. 3 is an illustration of a user creating a CAD model of anautomobile part, according to some embodiments;

FIG. 4 is an illustration of a conventional method for performing solidmodeling Boolean operations, according to some embodiments;

FIG. 5 is an illustration of a conventional B-rep solid model of anactual automotive part, according to some embodiments;

FIG. 6 is an illustration of how a conventional B-rep model mayintroduce gaps that may hinder animation of a model, according to someembodiments;

FIG. 7 is an illustration of a conventional faceted quadrilateral meshmodel as compared to the watertight B-rep model, according to someembodiments;

FIG. 8 is a graphical illustration of the watertightCAD method,according to some embodiments;

FIG. 9 is a flowchart of the watertightCAD method indicating five steps,including the three core technical steps, used to transform atraditional B-rep solid model into a watertight model, according to someembodiments;

FIG. 10 is a flow chart illustrating the algorithmic steps involved inthe watertightCAD methodology that may be performed depending on whetherpre-SSI or post-SSI input data is received, according to someembodiments;

FIG. 11 is a flow chart illustrating the steps of the preprocessingalgorithm when pre-SSI input data is received, according to someembodiments;

FIG. 12 is a flow chart illustrating the steps of the preprocessingalgorithm when post-SSI input data is received, according to someembodiments;

FIG. 13 is a flowchart diagram illustrating the steps involved toaccomplish parameter space analysis for the pre-SSI algorithm, accordingto some embodiments;

FIG. 14 is a flowchart diagram illustrating the steps involved toaccomplish parameter space analysis for the post-SSI algorithm,according to some embodiments;

FIG. 15 is a flowchart illustrating the steps involved in handling theparametrization change, according to some embodiments;

FIG. 16 is a flowchart illustrating the steps involved in performingreparameterization, according to some embodiments;

FIG. 17 is a flowchart illustrating the steps involved in performingreconstruction, according to some embodiments;

FIG. 18 is a flowchart illustrating the optional steps involved inperforming postprocessing on the reconstructed watertight surfaces,according to some embodiments;

FIG. 19 is a table identifying C_(PSi) types based on boundingsignificant point pair types, according to some embodiments;

FIG. 20 is an illustration of how each maximal contiguous group ofsegments of the same type may be combined to form a cluster (i.e.,interval) of the same type, according to some embodiments;

FIGS. 21A-21C are illustrations of isoparametric curve sampling anddivision based on the model space curve, according to some embodiments;

FIG. 21D is an illustration of the linear reparameterization based onthe sub-isocurves shown in FIG. 21C, according to some embodiments;

FIG. 21E is an illustration of a non-linear reparameterization scheme,using the sub-isocurves of FIG. 21C, according to some embodiments;

FIGS. 22A-22C illustrate the structural components of a reparametrizedparametric surface, according to some embodiments;

FIGS. 23A-23E further illustrate the structural components of areparametrized parametric surface, according to some embodiments;

FIGS. 24A-B is an illustration of two spline surfaces S₀ and S₁ thatintersect in model space, according to some embodiments;

FIG. 25A illustrates an example of the result of a solid modelingBoolean operation with an SSI operation on spline surfaces S₀ and S₁shown in FIGS. 24A-B, according to some embodiments;

FIG. 25B is a blowup illustration of area 200 detailing thediscontinuous gap-replete fashion of the Boolean operation performed asshown in FIG. 25A, according to some embodiments;

FIGS. 26A-C illustrate the details of the result of the SSI operationshown in FIG. 25A in model space and the various parameter spaces,according to some embodiments;

FIG. 27A is an illustration of blowups of areas 300, 500, and 600 forspline surface S₀, with details, according to some embodiments;

FIG. 27B is an illustration of blowups of areas 300, 700, and 800 forspline surface S₁, with details, according to some embodiments;

FIG. 27C is an illustration of blowups of areas 300 and 400, withdetails, along with a detail of the knot refined curve C_(MS), accordingto some embodiments;

FIG. 28 is an illustration of a detail of the knot refined curve C_(MS)with sample points shown, according to some embodiments;

FIGS. 29A-D is an illustration of isocurve sampling of surfaces S₀ andS₁, with both versions of untrimmed and trimmed isocurves at C_(MS),according to some embodiments;

FIGS. 30A-D is an illustration of an example of linear isocurvereparameterization for isocurve sampling of surfaces S₀ and S₁,according to some embodiments;

FIG. 31A is an illustration of isocurve sampling of surface S₀ with itsparameter space, according to some embodiments;

FIG. 31B is an illustration of a reconstructed T-spline surface {tildeover (S)}₀ with its respective parameter space, according to someembodiments;

FIG. 31C is an illustration of a reconstructed B-spline surface {tildeover (S)}₀ with its respective parameter space, according to someembodiments;

FIG. 31D is an illustration of isocurve sampling of surface S₁ with itsparameter space, according to some embodiments;

FIG. 31E is an illustration of a reconstructed B-spline surface {tildeover (S)}₁ with its respective parameter space, according to someembodiments;

FIG. 32A is an illustration of a T-spline surface {tilde over (S)} as aunion of {tilde over (S)}₀ and {tilde over (S)}₁ with its globalparameter space, according to some embodiments;

FIG. 32B is an illustration of a B-spline surface {tilde over (S)} as aunion of {tilde over (S)}₀ and {tilde over (S)}₁ with its globalparameter space, according to some embodiments;

FIG. 33 is an illustration of all the isocurves evaluated for theinternal knots of an arbitrarily defined surface, according to someembodiments;

FIG. 34 is an illustration of an extraction domain of an isocurve, andthe relationship between the isocurve's parametric and geometricrepresentations, according to some embodiments;

FIG. 35 is an illustration of isocurves split at the trim curve for theextraction domain depicted in FIG. 34, according to some embodiments;

FIG. 36 is an illustration of the construction of a linearreparametrized set of isocurves using isocurve linearreparameterization, according to some embodiments;

FIG. 37 is an illustration of a method for using a longest isocurve asthe model for the final form of the reparametrized knot vector,according to some embodiments;

FIG. 38 is an illustration of global parameter space reconstruction andreparameterization, according to some embodiments;

FIG. 39 is a table indicating segment type classification based onbounding significant point pair types, according to some embodiments;

FIG. 40 is an illustration of C₀ knots and corresponding significantpoint types that may be produced from downstream globalreparameterization and reconstruction operations, according to someembodiments;

FIG. 41 is another illustration of C₀ knots and correspondingsignificant point types that may be produced from downstream globalreparameterization and reconstruction operations, according to someembodiments;

FIG. 42 is an illustration of an example of the embedded extensions forthe significant point knot type 0, according to some embodiments;

FIG. 43 is an illustration of an example of the embedded extensions forthe significant point knot type 1, according to some embodiments;

FIG. 44 is an illustration of an example of the inserted extraordinarypoint technique, according to some embodiments; and

FIG. 45 is an illustration of an instance wherein the surface of asphere is modified from a quadball configuration to that of a standardsingle patch configuration with degenerate points at the poles,according to some embodiments.

While the features described herein are susceptible to variousmodifications and alternative forms, specific embodiments thereof areshown by way of example in the drawings and are herein described indetail. It should be understood, however, that the drawings and detaileddescription thereto are not intended to be limiting to the particularform disclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the subject matter as defined by the appended claims.

The term “configured to” is used herein to connote structure byindicating that the units/circuits/components include structure (e.g.,circuitry) that performs the task or tasks during operation. As such,the unit/circuit/component can be said to be configured to perform thetask even when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invokeinterpretation under 35 U.S.C. § 112(f) for that unit/circuit/component.

DETAILED DESCRIPTION OF THE EMBODIMENTS Acronyms

Various acronyms are used throughout the present application.Definitions of the most prominently used acronyms that may appearthroughout the present application are provided below:

BREP: Boundary Representation

NURBS: Non-Uniform Rational B-Spline

SSI: Surface-Surface Intersection

Terminology

The following is a glossary of terms used in this disclosure:

Memory Medium—Any of various types of non-transitory memory devices orstorage devices. The term “memory medium” is intended to include aninstallation medium, e.g., a CD-ROM, floppy disks, or tape device; acomputer system memory or random access memory such as DRAM, DDR RAM,SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash,magnetic media, e.g., a hard drive, or optical storage; registers, orother similar types of memory elements, etc. The memory medium mayinclude other types of non-transitory memory as well or combinationsthereof. In addition, the memory medium may be located in a firstcomputer system in which the programs are executed, or may be located ina second different computer system which connects to the first computersystem over a network, such as the Internet. In the latter instance, thesecond computer system may provide program instructions to the firstcomputer for execution. The term “memory medium” may include two or morememory mediums which may reside in different locations, e.g., indifferent computer systems that are connected over a network. The memorymedium may store program instructions (e.g., embodied as computerprograms) that may be executed by one or more processors.

Carrier Medium—a memory medium as described above, as well as a physicaltransmission medium, such as a bus, network, and/or other physicaltransmission medium that conveys signals such as electrical,electromagnetic, or digital signals.

Computer System—any of various types of computing or processing systems,including a personal computer system (PC), mainframe computer system,workstation, network appliance, Internet appliance, personal digitalassistant (PDA), tablet computer, smart phone, television system, gridcomputing system, or other device or combinations of devices. Ingeneral, the term “computer system” can be broadly defined to encompassany device (or combination of devices) having at least one processorthat executes instructions from a memory medium.

Processing Element—refers to various implementations of digitalcircuitry that perform a function in a computer system. Additionally,processing element may refer to various implementations of analog ormixed-signal (combination of analog and digital) circuitry that performa function (or functions) in a computer or computer system. Processingelements include, for example, circuits such as an integrated circuit(IC), ASIC (Application Specific Integrated Circuit), portions orcircuits of individual processor cores, entire processor cores,individual processors, programmable hardware devices such as a fieldprogrammable gate array (FPGA), and/or larger portions of systems thatinclude multiple processors.

Automatically—refers to an action or operation performed by a computersystem (e.g., software executed by the computer system) or device (e.g.,circuitry, programmable hardware elements, ASICs, etc.), without userinput directly specifying or performing the action or operation. Thusthe term “automatically” is in contrast to an operation being manuallyperformed or specified by the user, where the user provides input todirectly perform the operation. An automatic procedure may be initiatedby input provided by the user, but the subsequent actions that areperformed “automatically” are not specified by the user, i.e., are notperformed “manually”, where the user specifies each action to perform.For example, a user filling out an electronic form by selecting eachfield and providing input specifying information (e.g., by typinginformation, selecting check boxes, radio selections, etc.) is fillingout the form manually, even though the computer system must update theform in response to the user actions. The form may be automaticallyfilled out by the computer system where the computer system (e.g.,software executing on the computer system) analyzes the fields of theform and fills in the form without any user input specifying the answersto the fields. As indicated above, the user may invoke the automaticfilling of the form, but is not involved in the actual filling of theform (e.g., the user is not manually specifying answers to fields butrather they are being automatically completed). The presentspecification provides various examples of operations beingautomatically performed in response to actions the user has taken.

Configured to—Various components may be described as “configured to”perform a task or tasks. In such contexts, “configured to” is a broadrecitation generally meaning “having structure that” performs the taskor tasks during operation. As such, the component can be configured toperform the task even when the component is not currently performingthat task (e.g., a set of electrical conductors may be configured toelectrically connect a module to another module, even when the twomodules are not connected). In some contexts, “configured to” may be abroad recitation of structure generally meaning “having circuitry that”performs the task or tasks during operation. As such, the component canbe configured to perform the task even when the component is notcurrently on. In general, the circuitry that forms the structurecorresponding to “configured to” may include hardware circuits.

Various components may be described as performing a task or tasks, forconvenience in the description. Such descriptions should be interpretedas including the phrase “configured to.” Reciting a component that isconfigured to perform one or more tasks is expressly intended not toinvoke 35 U.S.C. § 112, paragraph six, interpretation for thatcomponent.

DETAILED DESCRIPTION

Computer-Aided Design (CAD) Systems

Modern computer-aided design (CAD) systems provide an environment inwhich users can create and edit curves and surfaces. The curves mayinclude spline curves such as B-spline curves, or more generally,Non-Uniform Rational B-Spline (NURBS) curves. The surfaces may includetensor product spline surfaces such as B-spline surfaces, or moregenerally, NURBS surfaces, or even more generally, T-spline surfaces.T-Spline surfaces allow the freedom of having T-junctions in the controlnet of the surface. Tensor product spline surfaces have certain verydesirable properties that make them popular as modeling tools, e.g.,properties such as: localized domain of influence of each surfacecontrol point on surface geometry; controllable extent of continuityalong knot lines; C^(∞) smoothness between knot lines; and the abilityto represent complex freeform geometry in a discrete manner that isintuitive for the user.

Two significant paradigms within computer-aided design with regards tothe geometric modeling of an object are surface modeling and solidmodeling.

In surface modeling, an object is represented simply as a set ofunconnected surfaces, without maintaining a model of the topologicalstructure corresponding to the relationships between the geometricfeatures of the object. Thus, while the user may design two trimmedsurfaces within an object that apparently intersect along a given curve,the CAD system does not explicitly record the topological or geometricrelationship between the trimmed surfaces. Thus, in surface modeling,surfaces are operated on as independent entities.

In solid modeling, the specification of an object requires thespecification of both topology and geometry, which are captured in adata structure referred to as a boundary representation (Brep, B-rep,b-rep or brep). From the topological point of view, a boundaryrepresentation includes, at minimum, faces, edges and vertices, andinformation regarding their interconnectivity and orientation. (Anobject includes a set of faces. Each face is bounded by a set of edges.Each edge is bounded by a pair of vertices.) From the geometric point ofview, the boundary representation includes surfaces, curves and points,which correspond respectively to the faces, edges and vertices of thetopological point of view. Thus, a boundary representation provides away to store and operate on a collection of surfaces, curves and pointsas a unified object. In addition to faces, edges and vertices, manysolid modelers provide additional objects to this data structure, suchas loops, shells, half-edges, etc.

Solid modeling may be used to create a 2-manifold or a non-manifoldobject. A set X is said to be 2-manifold (without boundary) whenever,for every point x∈X, there exists an open neighborhood in X thatcontains x and is homeomorphic to an open disk of the 2D Euclideanplane. The sphere S² and the torus T² are common examples of2-manifolds. A set X is said to be a 2-manifold with boundary whenever,for every point x∈X, there exists an open neighborhood in X thatcontains x and is homeomorphic to either an open disk of the 2DEuclidean plane or the half open disk given by {(x,y): x²+y²<1, andx≥0}. An example of a non-manifold object is the union of two planes.(This object fails the defining condition for a 2-manifold at each pointalong the intersection of the two planes.) Another example of anon-manifold object is the union of two spheres that touch each other ata single point of tangency.

Solid modeling may also be used to create a 3-manifold having a2-manifold as its boundary. Common examples of such a 3-manifold includea solid cube, a solid ball and a solid torus.

The process of designing a model for an object often involves thecreation of tensor product spline surfaces in a model space, and theapplication of Boolean operations to the surfaces (or to 3D “solids”that are defined by a 2-manifold B-rep of surfaces, as per above). TheBoolean operations require the calculation of intersections betweensurfaces in the B-rep. The resulting intersections are represented astrimmed surfaces. A fundamental problem with such Boolean operations isthat the trimmed surfaces do not meet in a C⁰ continuous fashion at thegeometric intersection of the original surfaces. The process involved indetermining the geometric location of intersection between two or moretensor product spline surfaces, as well as the creation of the resultingrepresentational geometric objects, is referred to as “surface-surfaceintersection” (SSI). Using current SSI technology, each trimmed surfaceexhibits an independent profile of small scale deviations from the truegeometric intersection. For example, a user of a CAD system may createspline surfaces S₀ and S₁ as shown in FIG. 1A, and apply one or moreBoolean operations to create the trimmed surfaces S_(0,TRIMMED) andS_(1,TRIMMED) shown in FIG. 1B. In this case, the user has specified theone or more Boolean operations to approximately remove from the domainof evaluation of surface S₀ the portion of S₀ that resides in front ofsurface S₁, and to approximately remove from the domain of evaluation ofsurface S₁ the portion of S₁ that resides above surface S₀. FIG. 1C is ablowup of the area 100 at the intersection of the trimmed surfacesS_(0,TRIMMED) and S_(1,TRIMMED), exhibiting some of the gaps at theintersection. Thus, Boolean operations typically violate the intuitiveexpectation that the trimmed surfaces should have a C⁰ continuousinterface along the geometric intersection of S₀ and S₁. This is clearlydemonstrated by the fact that the trim operations in surface modelingand Boolean operations in solid modeling do not alter the underlyingsurface representations. Instead, curves are altered and/or created soas to approximately update the domains of evaluation of the originalsurfaces.

The gaps are a consequence of a basic mathematical fact: the genericintersection of two low-degree polynomial surfaces is a 3D curve of veryhigh degree. For example, the solution to the intersection of twoarbitrary bicubic surfaces is a 3D curve of degree 324. While it istheoretically possible to compute the parameters of such a high degreeintersection curve based on the parameters of the two polynomialsurfaces, that computation would not be realizable within CADenvironments given the computing limitations as well as the practicalityof using the resulting high degree curve in practice. Thus, whenimplementing a Boolean operation, a conventional CAD system may:

(a) numerically compute a set of points SOP_(MS) that reside on (orsufficiently near) the surface-surface intersection in the 3D modelspace (and are scattered across the surface-surface intersection),and/or, compute a set of intersection-related points SOP_(PS0) in theparameter space of spline surface S₀, and/or, compute a set ofintersection-related points SOP_(PS1) in the parameter space of thespline surface S₁ (where the subscript “MS” is meant to suggest modelspace, and the subscript “PS” is meant to suggest parameter space);(b) generate a curve C_(MS) in the 3D model space, based on the set ofpoints SOP_(MS), where the curve C_(MS) approximates the surface-surfaceintersection, e.g., by interpolating the set of points SOP_(MS);(c) generate a trim curve C_(PS0) in the 2D parameter space of thespline surface S₀, based on the set of points SOP_(PS0), where the trimcurve C_(PS0) approximates the preimage of the surface-surfaceintersection under the spline map S₀, e.g., by interpolating the set ofpoints SOP_(PS0);(d) generate a trim curve C_(PS1) in the 2D parameter domain of splinesurface S₁, based on the set of points SOP_(PS1), where the trim curveC_(PS1) approximates the preimage of the surface-surface intersectionunder the spline map S₁, e.g., by interpolating the set of pointsSOP_(PS1).

When the Boolean operation is performed in the context of solidmodeling, the CAD system will update the boundary representation. Theupdate of the boundary representation may be performed in a variety ofways, e.g., depending on the data structure format of the boundaryrepresentation, as well as the solid modeling algorithm applied, e.g.,use of Euler operators or a variety of alternative operators. As anillustrative example in view of FIGS. 1A-1C, the boundary representationmay be updated by:

adding a new topological edge E and associating the curve C_(MS) withthe new topological edge E;

splitting an existing topological face F₀ corresponding to the splinesurface S₀ along a new topological coedge E₀ corresponding to the trimcurve C_(PS0), to obtain two subfaces F₀ ^(A) and F₀ ^(B);

associating the topological coedge E₀ with the edge E and with the trimcurve C_(PS0);

selecting one of the two subfaces F₀ ^(A) and F₀ ^(B) to be retained andthe other to be discarded, e.g., depending on the identity of theBoolean operation (intersection, subtraction, or union), and/or, on userinput;

replacing the face F₀ with its selected subface F₀ ^(selected), andassociating the spline surface S₀ with the selected subface F₀^(selected),

splitting an existing topological face F₁ corresponding to the splinesurface S₁ along a new topological coedge E₁ corresponding to the trimcurve C_(PS1), to obtain two subfaces F₁ ^(A) and F₁ ^(B);

associating the topological coedge E₁ with the edge E and with the trimcurve C_(PS1);

selecting one of the two subfaces F₁ ^(A) and F₁ ^(B) to be retained andthe other to be discarded, e.g., depending on the identity of theBoolean operation (intersection, subtraction, or union), and/or, on userinput;

replacing the face F₁ with its selected subface F₁ ^(selected), andassociating the spline surface S₁ with the selected subface F₁^(selected).

Observe that there are three available approximations to the truesurface-surface intersection: the trajectory of the model space curveC_(MS); the image S₀(C_(PS0)) of the trim curve C_(PS0) under the splinemap S₀; and the image S₁ (C_(PS1)) of the trim curve C_(PS1) under thespline map S₁. Each of those approximations exhibits an independentprofile of deviations from the true surface-surface intersectionSSI_(TRUE), and disagrees with the other approximations:C _(MS)≠SSI_(TRUE);S ₀(C _(PS0))≠SSI_(TRUE);S ₁(C _(PS1))≠SSI_(TRUE);S ₀(C _(PS0))≠C _(MS);S ₁(C _(PS1))≠C _(MS);S ₀(C _(PS0))≠S ₁(C _(PS1)).

In the above list, the notation X≠Y may be interpreted as meaning thatthe Hausdorff distance between set X and set Y is greater than zero. Thegaps between the trimmed surfaces may be due especially to theinequality S₀(C_(PS0))≠S₁ (C_(PS1)).

In the environment of surface modeling, there may be a separate modelspace curve CS_(MSk) for each surface S_(k) participating in theintersection. For example, surface S₀ may be trimmed relative to surfaceS₁, and surface S₁ may be separately trimmed relative to surface S₀.Each trimming operation may generate a corresponding set of intersectionpoints SOP_(MSk) in model space, and thus, result in a separate modelspace curve CS_(MSk). Thus, to the above multiplicity of setdisagreements, we should add, for each k:C _(MSk)≠SSI_(TRUE);C _(MSk) ≠S ₀(C _(PS0));C _(MSk) ≠S ₁(C _(PS1))

Therefore, a solid modeling Boolean operation on spline surfacesgenerally results in at least one model space curve and a set ofparametric trim curves per spline surface. Because these curvesrepresent separate approximations to the geometric intersection, gapsand openings are introduced along the geometric intersection.Furthermore, the editable surface features of the tensor product splinesurfaces (e.g. control points, degree, weights, etc.) are renderedun-editable and static.

The above described problem is known in computer-aided design as thetrim problem (or the SSI problem). The trim problem is considered to beone of the top five problems in the field, and many consider it to bethe most important problem. This problem affects all users of theresultant model, including the numerous downstream users andapplications of the model. There has been no general solution for thisproblem to date. The scope and weight of the problem may be summarizedwith an excerpt from, by one of the industry's leading experts on thetopic, Tom Sederberg [Sederberg 2008]:

-   -   “The existence of these gaps in trimmed NURBS models seems        innocuous and easy to address, but in fact it is one of the most        serious impediments to interoperability between CAD, CAM and CAE        systems. Software for analyzing physical properties such as        volume, stress and strain, heat transfer, or lift-to-drag ratio        will not work properly if the model contains unresolved gaps.        Since 3D modeling, manufacturing and analysis software does not        tolerate gaps, humans often need to intervene to close the gaps.        This painstaking process has been reported to require several        days for a large 3D model such as an airplane and was once        estimated to cost the US automotive industry over $600 million        annually in lost productivity. At a workshop of academic        researchers and CAD industry leaders, the existence of gaps in        trimmed-NURBS models was singled out as the single most pressing        unresolved problem in the field of CAD.”

Furthermore, the difficulty and long persistence of the trim problem isevidenced by the following assertions made by Christoph M. Hoffmann, onethe leading experts in the field of solid modeling, excerpted from[Hoffman 1989]:

-   -   “The difficulty of evaluating and representing the intersection        of parametric surface patches has hindered the development of        solid modelers that incorporate parametric surfaces. Roughly        speaking, the topology of a surface patch becomes quite        complicated when Boolean operations are performed. Finding a        convenient representation for these topologies continues to be a        major challenge.”

Currently there is no direct solution to the trim problem. Instead, mostindustries have developed alternative surface descriptions that attemptto avoid the issues with SSI. The alternative surface descriptionsapproximate the originally supplied surface splines. Examples ofalternative surface descriptions include subdivision surfaces andfaceted remeshing with geometric healing. As another example, T-splinesmight be constructed to represent a multi-patch NURBS surface geometry,given an appropriate set of conditions. Unfortunately, these alternativesurface descriptions introduce additional approximations, introduce manyother problems that did not previously exist in the original model, andeither do not allow for Boolean operations or provide for Booleanoperations in a manner that is unreliable and very difficult to use.

As noted above, without correction, the gaps between the trimmedsurfaces of object models would severely compromise downstream processessuch as engineering analysis, manufacturing, graphics rendering andanimation. For example, in a thermal analysis, the computed flow of heatbetween the trimmed surfaces would be disrupted by such gaps. In thecontext of 3D printing, the gaps would translate into locations wherematerial would be not deposited, and the structural integrity of theresulting printed object would be undermined. In graphics, the renderedimage would contain the appearance of cracks at the gaps. In animation,the objects and characters would separate when dynamically actuated.

As further noted above, the correction of such gaps between trimmedsurfaces unfortunately involves the exertion of intensive human labor.For example, prior to engineering analysis, the boundary representationmay be converted into a continuous mesh of triangles (or polygons).Tremendous human labor is typically required to heal the gaps byinjecting a myriad of small polygons to fill the area left betweensurfaces. The gaps may be so numerous and of such small scale that itmay be difficult to discover and correct all the gaps in a single passof human gap correction. Repeated cycles of analysis, machining,rendering, animation, depending on process of interest, may be dedicatedto nothing more than identifying locations of gaps not caught byprevious iterations of human gap correction, substantially decreasingthe time and budget available for meaningful analyses that would informoptimizations of the model design.

Existing solutions for preparing CAD models for analysis include healingsoftware (CADdoctor, Acc-u-Trans CAD, 3D Evolution, etc.) and meshingsoftware (Altair Hypermesh, CFX-Mesh, Ennova). These solutions fail tomitigate the workflow burden since they do not address the fundamentalmodel differences between disciplines. While they claim to expedite thesteps in the current CAD-CAE-CAM pipeline, the processes remainsemi-automated and require substantial amounts of users' time and skill.

Therefore, there is a vital need for mechanisms capable of performingBoolean operations on surfaces (or solid objects) in a boundaryrepresentation so that the resulting surfaces meet in a C⁰ continuousfashion at their geometric intersections. Furthermore, there exists avital need for mechanisms capable of reconstructing the surfaces in apreviously-created boundary representation so that the reconstructedsurfaces meet at their geometric intersections in a C⁰ continuous (i.e.,conformal) fashion. These mechanisms may enable the benefits oftechnologies such as isogeometric analysis (IGA) to be more fully andsystematically realized.

FIG. 2: Computer System

FIG. 2 illustrates one embodiment of a computer system 900 that may beused to perform any of the method embodiments described herein, or, anycombination of the method embodiments described herein, or any subset ofany of the method embodiments described herein, or, any combination ofsuch subsets.

In some embodiments, a hardware device (e.g., an integrated circuit, ora system of integrated circuits, or a programmable hardware element, ora system of programmable hardware elements, or a processor, or a systemof interconnected processors or processor cores) may be configured basedon FIG. 2 or portions thereof. Any hardware device according to FIG. 2may also include memory as well as interface circuitry (enablingexternal processing agents to interface with the hardware device).

Computer system 900 may include a processing unit 910, a system memory912, a set 915 of one or more storage devices, a communication bus 920,a set 925 of input devices, and a display system 930.

System memory 912 may include a set of semiconductor devices such as RAMdevices (and perhaps also a set of ROM devices).

Storage devices 915 may include any of various storage devices such asone or more memory media and/or memory access devices. For example,storage devices 915 may include devices such as a CD/DVD-ROM drive, ahard disk, a magnetic disk drive, a magnetic tape drive,semiconductor-based memory, etc.

Processing unit 910 is configured to read and execute programinstructions, e.g., program instructions stored in system memory 912and/or on one or more of the storage devices 915. Processing unit 910may couple to system memory 912 through communication bus 920 (orthrough a system of interconnected busses, or through a computernetwork). The program instructions configure the computer system 900 toimplement a method, e.g., any of the method embodiments describedherein, or, any combination of the method embodiments described herein,or, any subset of any of the method embodiments described herein, or anycombination of such subsets.

Processing unit 910 may include one or more processors (e.g.,microprocessors).

One or more users may supply input to the computer system 900 throughthe input devices 925. Input devices 925 may include devices such as akeyboard, a mouse, a touch-sensitive pad, a touch-sensitive screen, adrawing pad, a track ball, a light pen, a data glove, eye orientationand/or head orientation sensors, a microphone (or set of microphones),an accelerometer (or set of accelerometers), or any combination thereof.

The display system 930 may include any of a wide variety of displaydevices representing any of a wide variety of display technologies. Forexample, the display system may be a computer monitor, a head-mounteddisplay, a projector system, a volumetric display, or a combinationthereof. In some embodiments, the display system may include a pluralityof display devices. In one embodiment, the display system may include aprinter and/or a plotter.

In some embodiments, the computer system 900 may include other devices,e.g., devices such as one or more graphics devices (e.g., graphicsaccelerators), one or more speakers, a sound card, a video camera and avideo card, a data acquisition system.

In some embodiments, computer system 900 may include one or morecommunication devices 935, e.g., a network interface card forinterfacing with a computer network (e.g., the Internet). As anotherexample, the communication device 935 may include one or morespecialized interfaces for communication via any of a variety ofestablished communication standards or protocols or physicaltransmission media.

The computer system 900 may be configured with a software infrastructureincluding an operating system, and perhaps also, one or more graphicsAPIs (such as OpenGL®, Direct3D, Java 3D™).

Any of the various embodiments described herein may be realized in anyof various forms, e.g., as a computer-implemented method, as acomputer-readable memory medium, as a computer system, etc. A system maybe realized by one or more custom-designed hardware devices such asASICs, by one or more programmable hardware elements such as FPGAs, byone or more processors executing stored program instructions, or by anycombination of the foregoing.

In some embodiments, a non-transitory computer-readable memory mediummay be configured so that it stores program instructions and/or data,where the program instructions, if executed by a computer system, causethe computer system to perform a method, e.g., any of the methodembodiments described herein, or, any combination of the methodembodiments described herein, or, any subset of any of the methodembodiments described herein, or, any combination of such subsets.

In some embodiments, a computer system may be configured to include aprocessor (or a set of processors) and a memory medium, where the memorymedium stores program instructions, where the processor is configured toread and execute the program instructions from the memory medium, wherethe program instructions are executable to implement any of the variousmethod embodiments described herein (or, any combination of the methodembodiments described herein, or, any subset of any of the methodembodiments described herein, or, any combination of such subsets). Thecomputer system may be realized in any of various forms. For example,the computer system may be a personal computer (in any of its variousrealizations), a workstation, a computer on a card, anapplication-specific computer in a box, a server computer, a clientcomputer, a hand-held device, a mobile device, a wearable computer, acomputer embedded in a living organism, etc.

Reconstruction of Spline Surfaces to Integrate Surface-SurfaceIntersection and Provide Inter-Surface Continuity

Current technologies employ methods that only allow for one-wayconversion of a CAD model, while also introducing additionalcomplications, data structures, and approximations. Such methods includehealing/translation applications, polytope meshing software(quadrilaterals, triangles, etc.), and conversions to alternativeformats such as subdivision surfaces. Embodiments described herein mayproduce objects in the same format as the original model (i.e., asfreeform surfaces, not polygonal approximations), making them availablefor editing in the user's native CAD software. Current technology usedby existing CAD, meshing, and healing/translation software applicationsdoes not allow users to repair models that retain editable properties.

CAD may be applicable to an increasing number of industries, and it isexpected that the CAD industry will continue to expand significantly inthe coming years. This growth is anticipated due to the need forenhanced product visualization and increased design process efficiency.

The CAD industry may generally be categorized into four distinct marketsegments: design, engineering (analysis), manufacturing, and animation.These four segments are served by vertical CAD products, limitinghorizontal interoperability between applications and making it difficultfor engineers and designers to use a single model for the purposes ofanalysis, manufacturing, and graphics. Some companies exist solely toheal models and deliver models to customers with suitable formats thatmeet the constraints of current CAD representations and limitationsthroughout the modeling pipeline.

Modern engineering practice may rely on CAD systems to create geometricrepresentations of components and assemblies. CAD geometry may becomethe input to CAE (FEA, CFD, etc.), CAM (CNC tool path generation,additive manufacturing, etc.), animation and graphics, inspection, etc.In a typical process, a designer may create a model of a componentand/or system and then an engineer may use this CAD model as input forconverting to an analysis-suitable model for use in a CAE program. Thefabricator may also convert the CAD model independently to theirrequirements so as to analyze manufacturability with a CAM program. FIG.3 illustrates a user creating a CAD model on a computer, which may thenbe used for construction of a corresponding vehicle part. The overalldesign process may require many iterations of redesign as results arefed back to modify the original CAD model, and each cycle may require amodel translation in the design-analysis-manufacturing workflow withdifferent, specialized users involved in arriving at a satisfactoryfinal design.

To represent the highly freeform nature of complex objects, non-uniformrational B-spline (NURBS) surfaces are commonly used geometricrepresentations in CAD systems. NURBS surfaces are mappings from basisfunctions in a UV-parameter space to the 3D model space, where the usersees their intended geometric design. The most natural and common methodof building unique, multifaceted models in CAD systems is to performBoolean operations, or unions, intersections and differences, on thefreeform NURBS surfaces to build up the distinct details of each design.Numerical solutions of the surface-surface intersections (SSI) in 3Dmodel space may be necessary as the exact intersections of thesesurfaces may be of a polynomial degree and complexity too large for CADsystems to realistically handle in software or to be of practical use bythe user for further shaping. An auxiliary topological procedure maythen be used to create an ancillary data structure, called a boundaryrepresentation (B-rep), providing an independent and non-geometric meansof maintaining soundness and validity of the solid.

FIG. 4: Conventional Solid Modeling Technique

FIG. 4 is an illustration of a conventional method for performing solidmodeling Boolean operation, which shows the approximation of the B-repsurface-surface intersection. In conventional methods, the underlyingNURBS surfaces are typically not altered or updated to reflect the truesurface intersections. Rather, an intersection curve may be associatedwith the surfaces where the SSI occurs by use of the B-rep datastructure. Because the underlying surface geometry is not updated, thesenumerical intersection algorithms may introduce gaps and openings in thegeometry at the intersections Further, because the B-rep data structureseparates topology from the geometry, the resulting non-watertightsurfaces may be rendered individually un-editable and static. Thegraphics card hides from the user the fact that the surface definitionsare never updated, rather, simply removed from rendering by the graphicsdisplay in the CAD application.

FIG. 5: Conventional B-Rep Solid Model of an Actual Automotive Part

FIG. 5 illustrates a conventional B-rep solid model of an actualautomotive part (left and right images are of the exact same model).FIG. 5 shows (left) the graphic display within the CAD applicationrendered by the graphics card and (right) the 1200+ surface definitionscontained in the data structure. Note the discontinuity and independentorder of the surface geometry.

A significant weakness of the current intersection paradigm is that theunderlying NURBS surfaces are not updated. As shown in FIGS. 4 and 5,the CAD user may see the intent of the operation performed, not becausethe object definition has been updated to represent the joined geometricconfiguration, but because the graphics card provides a means of notshowing the unwanted portions of the trimmed surfaces (generally througha rasterized mesh). Although this process has worked for displaying CADmodels while the user completes their design, this strategy does notwork for those using the CAD model in CAE, CAM, animation, orinspection. For example, FIG. 6 illustrates how gaps and a lack ofcontinuity in a standard B-rep model may hinder animation of a model. Incontrast, a gap free, watertight continuous model may be moreeffectively animated. Users may desire watertight, gap-free models,making a standard CAD model unusable without further processing. Assuch, an immense effort may be required to heal the model to make itgap-free. This cleanup process may be costly, time consuming andfrustrating for those inheriting the CAD model. Each iteration of thedesign process described above may require the engineer to repair gapsbetween the disparate surfaces introduced by Boolean operations andtranslate the geometric model from the native CAD B-rep representationto a valid analyzable form, typically a polygonal mesh. This operationmay require repetitive, semi-automated and/or manual examination at finelevels of detail to identify and repair the flaws. This results in aone-way design process hindering design feedback, optimization, andinformation integrity. Dr. Thomas Sederberg, a seminal researcher andexpert in CAD, described both the subtlety and gravity of the problem:

“The existence of these gaps in trimmed NURBS models seems innocuous andeasy to address, but in fact it is one of the most serious impedimentsto interoperability between CAD, CAM and CAE systems . . . . Since 3Dmodeling, manufacturing and analysis software does not tolerate gaps,humans often need to intervene to close the gaps. This painstakingprocess has been reported to require several days for a large 3D modelsuch as an airplane.”

FIG. 7—Modeling for Computational Analysis

Embodiments presented herein may address a core obstacle in the fieldthat costs time and money in lost productivity due to repairing modelsbuilt on B-rep Boolean operations. FIG. 7 illustrates a conventionalfaceted quadrilateral mesh model as compared to the watertight B-repmodel according to embodiments presented herein. Non-watertight modelsmay limit the abilities of, e.g., CAE, CAM, graphics technologies,additive manufacturing/3D printing, metrology and inspection, as theyall may need to heal and rebuild, or mesh, the model to mitigate gaps inthe CAD geometry. Entire branches of technology and industry have beenset up to deal with this expensive and limiting problem, wheredifferences in B-rep solid definitions across applications may break themodels' solid integrity upon file translation. The problem is sopervasive that many companies specializing in healing CAD models havebeen created to serve the market. Studies have shown that engineersspend a significant portion of their time repairing CAD models, and theamount of time spent repairing CAD models continues to grow as thegeometry of models become increasingly complex. This is one of theleading drivers in the development and popularity of isogeometricanalysis (IGA) within engineering analysis, a method which seeks to makeanalysis more CAD friendly. By eliminating gaps in models, embodimentspresented herein may remove a substantial roadblock and empower IGA as acommercial technology.

Embodiments herein present a watertightCAD methodology that providesgeometric and topological reconstruction designed to enhance currentB-rep modeling. In some embodiments, output surfaces are generated withsurface edges (isocurves) that meet continuously along the SSI curve,creating a gap-free result. Furthermore, the B-rep data may be updatedto furnish information about the geometric objects' parameter spaces,hence integrating the geometry and topology divide. This may allow thesurfaces of the solid model to become editable in the CAD application, ahuge advantage for modelers needing to make local updates to thegeometry. The watertight models may be analysis-suitable within the IGAframework, with no additional healing or meshing required.

In some embodiments, subsequent to outputting a modified, watertightCADmodel, the method may proceed by executing an engineering analysis onthe modified model to obtain data predicting physical behavior of anobject described by the model. In some embodiments, the engineeringanalysis may comprise an isogeometric analysis. In some embodiments, themethod may proceed to direct a process of manufacturing an objectdescribed by the modified model. In some embodiments, the method mayproceed to generate an image of an object based on the modified model.In some embodiments, e.g., in an animation application, the method mayproceed to generate a sequence of animation images based on the modifiedmodel and display the sequence of animation images.

FIG. 8-9—Watertight CAD Method and Flowchart

FIG. 8 is a graphical illustration of the watertightCAD method,according to some embodiments. As shown, some embodiments presentedherein create new NURBS surfaces, based on the original surfacegeometry, that integrate the surface-surface intersection informationcompletely within a simple NURBS surface description, thus producing amodel that matches the geometric and topological design intent. Thesurface edges may meet at respective isoparametric boundaries(isocurves). In some embodiments, this solution may solve the trimproblem natively, cleanly, and in a way that produces objects in thesame form as that of the original, hence it may be a closed-formoperation. This may make the technology employable in any current CADsystem using standard software kernel functionality. As such, the NURBSsurface objects that are output may retain editable properties (controlpoints, weights, knots, etc.), which may be unavailable within thepresent B-rep solid modeling paradigm. In some embodiments, theretention of editable properties may be achieved without introducing anynew data structures on the new NURBS surface. The result may not be apolygonal, mesh or STL approximation. Some embodiments may provide along-awaited alternative to B-rep solid modeling Boolean operations, andmay supply a solid model for the CAD user that may be accurate to modeltolerance.

FIG. 9 is a flowchart of the watertightCAD method indicating five steps,including the three core technical steps, required to transform atraditional B-rep solid model into a watertight model, according to someembodiments. In some embodiments, the waterightCAD method may beperformed as part of a Boolean operation in a computer-aided design(CAD) software system. These steps are illustrated graphically in FIG.8, where the basis functions S₁ and S₂ are plotted in the (u,v)parameter space. Notice that initially, the S₁ and S₂ basis functionsare each expressed in terms of their own unique parameters, (u₁,v₁) and(u₂,v₂), respectively. The reparameterization step then describes a newglobal set of basis functions {tilde over (S)} that are expressed interms of a new global parameter space (ũ, {tilde over (v)}).

The watertightCAD algorithm may begin by inputting a CAD file comprisinga first and second input surface associated with the solid model. Eachof the first and second input surfaces may be described in their ownparameter space domains, and may be non-uniform rational basis splines(NURBS). For example, the first input surface may be described within afirst parameter space domain and the second input surface may bedescribed in a second parameter space domain. A parameter space domainmay be understood to be distinct from a parameter space in that aparameter space domain is constrained by a knot vector. The first andsecond input surfaces may cross in space along at least one intersectioncurve, and methods herein may seamlessly connect the input surfacesalong their intersection.

Subsequent to the input of a CAD file, and as explained in furtherdetail below, preprocessing may be optionally performed on the CAD fileat 100. Subsequently, three core technical steps, 200, 300, and 400 maybe performed, which may be described as parameter space analysis,reparameterization, and reconstruction, respectively. 200, 300, and 400may be vital functions in transforming a standard B-rep solid model to awatertight representation.

Parameter space analysis 200 may comprise identifying and storing amodel space trim curve that approximates a geometric intersection of thefirst and second input surfaces. Characteristic points may be identifiedalong the model space trim curve, to aid in the reparameterizationprocess.

Reparameterization 300 may comprise reparametrizing the first and secondparameter space domains into a single global parameter space domain. Inother words, the first and second parameter space domains may bereparametrized into a common third parameter space domain. The thirdparameter space domain may be determined based on the model space trimcurve. In other words, the third parameter space domain may bedetermined based on the geometric intersection of the first and secondinput surfaces. In some embodiments, reparametrizing may compriseconstructing a series of isocurves based on the identifiedcharacteristic points of the model space trim curve. The constructedseries of isocurves may be comprised within the third parameter spacedomain.

Reconstruction 400 may comprise constructing first and second outputsurfaces that are each described in the third parameter space domain.The first and second output surfaces may be constructed such that atleast a portion of each of their boundaries coincides with the modelspace trim curve. In some embodiments, a portion of the boundary of eachof the first and second output surfaces may be constructed to coincidewith the constructed series of isocurves comprised within the thirdparameter space domain. The concatenation of the first and second outputsurfaces may therefore be watertight along this portion of each of theirboundaries.

These steps may be required for either the pre-SSI or post-SSIalgorithms, albeit the specific parameter space processing sub-steps mayvary based on the particular algorithm. At 500, postprocessing may beoptionally performed on the first and second output surfaces. Finally,the first and second output surfaces may be output as modified surfacesassociated with the solid model.

FIG. 10: Pre-SSI and Post-SSI Algorithms

FIG. 10 is a flow chart illustrating the algorithmic steps involved inthe watertightCAD methodology that may be performed depending on whetherpre-SSI or post-SSI input data is received. The pre-SSI algorithm mayassume the user has two B-reps they wish to perform a solid modelingBoolean operation on. The post-SSI algorithm may assume the user has avalid B-rep model that contains the results of previously computed,standard CAD solid modeling Boolean operations. In other words, pre-SSIinput data may comprise two B-reps upon which it is desired to performsolid modeling Boolean operations. Post-SSI input data may comprise avalid B-rep model that contains the results of previously computed,standard CAD solid modeling Boolean operations. For both the pre-SSI andpost-SSI algorithms, geometric input data may be stored describing afirst and second input surface associated with the model underconsideration. Each of the first and second input surfaces may bedescribed in a first and second respective parameter space domain.

The input for the Post-SSI algorithm may be a boundary representationcreated using one or more (typically many) solid modeling Booleanoperations on spline surfaces. (The spline surfaces may include tensorproduct spline surfaces such as NURBS surfaces and/or T-Splinesurfaces.) In one embodiment, the boundary representation may beprovided in a computer-aided design (CAD) file. In another embodiment,the Post-SSI algorithm may operate as part of the kernel of a CADsystem, in which case the boundary representation may have a specializedinternal format.

There are many different ways to realize a boundary representation interms of a data structure. (Different CAD packages have different datastructure realizations.) Furthermore, there are many different fileformats used to represent the brep data. For execution of the Post-SSIalgorithm, the data structure type and/or data file format of the CADfile need not be of any specific standard, whether a product based typeor interoperability standard. Some examples of data file formatsinclude, but are not limited to, ACIS (*.sat, *.sab), STEP (*.stp,*.step), Rhino (*.3dm, *.3dmbak), etc.

The Post-SSI algorithm may operate on a boundary representation of anobject, and not on alternative representations such as polytope meshes(structured or unstructured), constructive solid geometry (CSG) data,and implicit geometric representations.

Preprocessing

In some embodiments, to enable processing, necessary information may beparsed from the data structure in an optional preprocessing step 100.For the pre-SSI algorithm, at 110, this specifically consists ofidentifying the intended surfaces involved in the intersection, alongwith associated topological elements. For the post-SSI algorithm, at120, this specifically consists of parsing the topological datastructure to determine the surfaces involved in the intersection, thetrim curves stored in the B-rep, and the topological elements indicatingthese relationships. For example, for both pre-SSI and post-SSIalgorithms, preprocessing may comprise storing a model space trim curveassociated with the first and second input surfaces, wherein the modelspace trim curve approximates a geometric intersection of the first andsecond input surfaces. In some embodiments, the model space trim curvemay comprise sequential segments of isocurves in a common thirdparameter space domain, as discussed in greater detail below.

FIG. 11: Preprocessing for the Pre-SSI Algorithm

For embodiments wherein pre-SSI input data is received, thepreprocessing algorithm may begin at 111, wherein the specifictopological elements (faces, loops, edges, vertices) involved in the SSIoperation may be identified.

Subsequently, at 112, faces may be identified from the identifiedsurfaces of 111.

At 113, the faces may be used to identify associated topologicalelements. For example, loops, edges and vertices may be identified fromthe identified faces of 112.

FIG. 12: Preprocessing for the Post-SSI Algorithm

For embodiments wherein post-SSI input data is received, thepreprocessing algorithm may proceed differently from when pre-SSI inputdata is received. In particular, at 121, specific topological elements(faces, loops, edges, vertices) involved in the SSI operation may bedetermined.

At 122, from the determined edges of 121, 3D model space trim curves maybe identified. For example, the model space trim curves may approximategeometric intersections of the input surfaces. In some embodiments, themodel space trim curve may be constructed based on the characteristicpoints of the first and second input surfaces and the geometricintersection of the first and second input surfaces. In someembodiments, the model space trim curve may be received as an outputfrom a previous CAD Boolean operation on the first and second inputsurfaces.

At 123, 2D trim curves in respective UV parameter spaces may beidentified from the determined loops of 121.

Finally, at 124, NURBS surfaces may be identified from the determinedfaces of 121.

In some embodiments, these tasks may be accomplished with suitable callsto a native CAD application programming interface (API). Thepreprocessing steps described in 110 and 120 are standard tasks in anyCAD solid modeler, and are well known in the art.

Parameter Space Analysis

After performing a proper topological analysis of the B-rep model toisolate critical elements, in some embodiments, a subsequent step may beto analyze the trim curve data in the individual parameter spaces inorder to provide the requisite data to define a global UV parameterspace. This data may comprise classified trim curve segments reorganizedin the B-rep structure. As illustrated in FIG. 10, the parameter spaceanalysis step 200 may proceed differently depending on whether pre-SSI(210) or post-SSI (220) input data is received.

The output from either algorithm's (pre-SSI or post-SSSI) parameterspace analysis is a model space trim curve yielding the knot vector andisocurve sampling points that are desirable for completion of thewatertight surface operations. In some embodiments, the model space trimcurve comprises sequential segments of isocurves in a global parameterspace domain.

FIG. 13—Parameter Space Analysis for the Pre-SSI Algorithm

FIG. 13 is a flowchart diagram illustrating the steps involved toaccomplish parameter space analysis for the pre-SSI algorithm, accordingto some embodiments. In the pre-SSI algorithm, the intersecting surfacesmay be used to create a model space trim curve optimized for theoperations in the downstream reparameterization and reconstructionsteps.

At 211, for each surface, all of the characteristic points may be solvedfor as topologically ordered point sets in respective parameter spaces.Characteristic points may be common in any robust SSI operation,identified by root solving.

At 212, from the ordered point sets, classification and clusteringoperations may be performed. Segments may be classified based on thecharacteristic point type (derivatives with respect to dominantparametric direction) and may indicate whether they produce u-type orv-type edges, by use of a lookup table.

At 213, surface knots may be added to ordered point sets through clusterinformation.

At 214, point sets may be projected and exchanged for each surface. Thetopology may be checked and corrected. Model space subsets may beformed.

At 215, an intersection curve may be created by various conventionalmethods. The intersection curve may interpolate the model space pointset, forming a knot vector.

At 216, the model space points may be evaluated from curve Grevilleabscissae. Respective surface intersection points may be determinedthrough various conventional methods.

FIG. 14: Parameter Space Analysis for the Post-SSI Algorithm

FIG. 14 is a flowchart diagram illustrating the steps involved toaccomplish parameter space analysis for the post-SSI algorithm,according to some embodiments. In the post-SSI algorithm, the parameterspace and model space trim curves supplied by the conventional SSIoperation performed prior to input may be used for the operations in thedownstream reparameterization and reconstruction steps.

In the post-SSI algorithm, since the Boolean operation has beenconventionally performed already by a CAD application, there exists trimcurve data to analyze, both in parameter space as well as in modelspace. Because of this, a first step may be to analyze the trim curvedata in the individual parameter spaces in order to provide therequisite data to define a global UV parameter space. This data maycomprise classified trim curve segments reorganized in the B-repstructure.

Parameter space analysis for the post-SSI algorithm begins at 221,wherein specific points on trim curves and associated data aredetermined. In some embodiments, this may comprise developing a rootsolver for computing trim curve derivatives. The determined specificpoints on trim curves may include: characteristic points (or significantpoints) based on derivative values (tangent vectors), surface knotlocations, as well as trim curve knot data. Characteristic points may becommon in any robust SSI operation, solved by root solving, while thesurface and curve knots may be particular to embodiments describedherein.

At 222, segments between characteristic points found in 2.1 may beidentified and classified. Segments of the model space trim curve may beidentified and classified based on the characteristic points, whereineach identified segment comprises an isocurve in the third parameterspace domain, wherein said classification determines a parameter in thethird parameter space domain that has a constant value along theisocurve. Segments may be classified based on the characteristic pointtype (derivatives with respect to dominant parametric direction) and mayindicate whether they produce u-type or v-type edges. This functionalitymay be missing from current CAD applications and may be enabled by useof a lookup table.

Finally, at 223, the B-rep data structure may be updated by conventionaltopological operations (i.e. Euler operators) allowing subdivision ofthe segmented trim curves.

Reparameterization

During reparameterization, a new parameter space may be defined toreproduce the original surface geometry within model tolerance whilemaintaining surface continuity and explicitly integrating SSI data. Forexample, reparameterization may comprise reparametrizing the first andsecond parametric into a common third parameter space domain based onthe previously determined model space trim curve. This procedure mayutilize isocurves sampled in sets from the source surfaces (e.g., thefirst and second input surfaces) which may be then collectivelyreparametrized to define a newly constructed (third) global parameterspace. In other words, reparametrizing may comprise, for each of a setof isocurve sampling points, sampling an isocurve, wherein each sampledisocurve intersects another perpendicular isocurve at the isocurvesampling point. Additionally, the reparameterization function may havemany degrees of freedom that may require definition as a nonlinearmapping between parameter spaces in order to produce an operation thatis smooth, continuous, and of minimal complexity.

In some cases, transforming UV parameter space coordinates from a groupof independent, trimmed surfaces in a B-rep solid model to a singlewatertight definition may be challenging because there is no uniquesolution. Based on surface reconstruction methodology leveragingisocurve data, embodiments presented herein employ reparameterizationoperations established for individual NURBS curves. A more directmathematical formulation may use a composite function. The resultingisocurves of this composition may become complicated, arbitraryfunctions of an unpredictable form that cannot be guaranteed to berepresentable in a CAD system. Alternatively, reparameterizationfunctions may be utilized that are themselves B-splines, along with aprocedure to create a NURBS representation of the resulting isocurves.Such concepts are well documented and procedures for creating theserepresentations may be found in references on NURBS. Embodimentspresented herein introduce a novel approach by applying these methods torepair gaps in trim curves.

It may be difficult in some cases to develop functions forreparametrizing full sets of isocurves. For example, reparameterizationmay not be simply extending current theory by means of a batch process,but involve techniques replete with their own unique constraints anddegrees of freedom. An essential feature of embodiments presented hereinis to correctly map the parameter space of a family of isocurves to anew domain of a specified knot vector while maintaining the exactgeometric trace in model space. The parametric conversion may requiredefining reparameterization functions whose construction are central tothe success of the watertight method.

In prior implementations, the possibility of error and incompatibilityis introduced if independent isocurve reparameterizations are notmonitored and regulated. Embodiments presented herein may constrain thereparameterization task to maintain a valid output. In some embodiments,a reparameterization surface function may be defined as a bivariatetensor product spline surface, fully mapping the source to target UVparameter spaces. This function may be strictly monotonic in thedirection of the reparametrized variable, may interpolate the pointsdefined from pairs of source and target knot values for each isocurve,and the reparameterization function itself may be built from the knotvectors of the target parameterization (e.g., the Greville criteria).Although the reparameterization function may be linear or piecewiselinear, a nonlinear description may be desirable. However, a nonlineardescription may be harder to construct as there are an infinite numberof candidates that fulfill the above criteria. The method ofconstruction determined may ultimately contribute attributes to thecorresponding isocurves used for reconstruction, so correctly definingthe reparameterization function may be very important.

Tasks associated with reparameterization may be comprised of two mainsteps: parameterization change handling, followed by isocurvereparameterization and global parameter space reconstruction. Prior tothe actual reparameterization step, it may be desirable to handlechanges in parameterization. Specifying extraordinary points (E.P.s) inunstructured meshes may be identified in this manner. E.P.s may beintroduced by use of an intermediate trim curve for the purposes ofmodifying the local topology.

FIG. 15: Parameterization Change Handling

FIG. 15 is a flowchart illustrating the steps involved in handling theparametrization change, according to some embodiments. At 311,parameterization change handling is performed based on specification ofthe handling type: E.P., embedded extensions, or C⁰. Parameterizationchange handling may also be performed based on a null algorithm, whichmay be a pass-through algorithm that does nothing.

At 312, for E.P. or embedded extension handling types, intermediate trimcurves are constructed. The intermediate trim curves may be comprisedwithin at least one of the first and second input surfaces. Theextraordinary point insertion algorithm may introduce an intermediatetrim curve that intersects the model space trim curve and is orthogonalto the model space trim curve at the intersection. The embeddedextensions algorithm may perform tangent extension without introducingextraordinary points.

At 313, the rest of the reparameterization and reconstruction operations(e.g., steps 320 and 400) may be performed based on these intermediatetrim curves before they are performed for the trim curve originallyidentified for processing in the parameter space analysis, but for thisupdated surface based on the parameterization change handling.

FIG. 16: Reparameterization:

FIG. 16 is a flowchart illustrating the steps involved in performingreparameterization, according to some embodiments. Subsequent to theparameterization change handling steps of 311-313, the following may beperformed.

At 321, isoparametric curves (isocurves) may be sampled from theoriginal surfaces at locations determined in 200.

At 322, a bivariate reparameterization function may be specified by userinput or by pre-calculated criteria, defining a global parameter space.The bivariate reparameterization function may be based on aninterpolation strategy, and the interpolation points may be defined frompairs of isocurve source and target knot values. In some embodiments, itoptimization characteristics of the tensor-product reparameterizationsurface function (curvature, continuity, knot complexity) may bedetermined. These criteria may serve as critical metrics in evaluatingperformance.

At 323, each isocurve may be reparametrized based on thereparameterization function. In some embodiments, the reparameterizationfunction may be a basis spline function that maps each of the first andsecond parameter spaces into the common third parameter space.

In some cases, an additional important aspect of the reparameterizationstep may be the need to handle extraordinary points (E.P.s) inunstructured meshes. Embodiments herein may allow for specifying E.P.sby use of an intermediate trim curve for the purposes of modifying thelocal topology. This has traditionally been shown to be effective instandard tests (e.g., hole-in-the-plate), yet may require interventionto specify the bounds of the operation for general modeling. In order toavoid such manual specifications, we may identify a natural andrepeatable transition in parameterization for arbitrary models that caneasily be implemented in our programming framework. To accomplish thisobjective the following tasks may be completed:

3.2.1. Inspect and record E.P. reparameterization bounds through focusedanalysis of models furnished by industrial partners. The bounds maypresent themselves at characteristic points identified near E.P.s thatspread to the surrounding parameterization.

3.2.2. Formalize and validate the measured criteria from task 3.2.1 withrespect to theory. The criteria may be aligned to the current researchfield on E.P.s.

3.2.3 Improve theory from 3.2.2 by application to models in 3.2.1.

FIG. 17: Reconstruction

FIG. 17 is a flowchart illustrating the steps involved in performingreconstruction, according to some embodiments. With a global parameterspace created from the input B-rep SSI information, the model spacegeometry may be reconstructed to correctly represent the intersectionrequirements. In implementations of the present invention, it may bedesirable for the surfaces to meet in a C⁰ continuous fashion acrosstheir geometric intersections, and for the original surface geometry tobe maintained to model tolerance. In other words, the algorithm mayproceed to construct first and second output surfaces, wherein the firstand second output surfaces are described in the third parameter spacedomain. The first and second output surface may be constructed such thatat least a portion of the boundary of each of the first and secondoutput surfaces coincides with the model space trim curve. In someembodiments, the first and second output surfaces may be constructedwithin a predetermined error tolerances of the first and second inputsurfaces, respectively. In some embodiments, the predetermined errortolerance may be user-defined.

In some embodiments, reconstruction may begin at 401, whereinreparametrized isocurves may be grouped based on target reconstructionsurface domains.

At 402, control points of the individual isocurves may be arranged inmatrices.

At 403, reconstructed surface control points may be solved for based onisocurve control points.

At 404, from the solved control points of the reconstructed surfaces,along with the global parameter space, reconstructed surfaces may bedefined.

FIG. 18: Postprocessing

FIG. 18 is a flowchart illustrating the optional steps involved inperforming postprocessing on the reconstructed watertight surfaces,according to some embodiments. In some embodiments, to complete theclosed operation, it may be desirable for the model components to bereorganized within the standard B-rep data structure and returned to thenative CAD application. In some embodiments, repopulating the B-rep datastructure may proceed with the following tasks:

At 501, the topological elements (faces, loops, edges, vertices) may beupdated.

At 502, the disparate surfaces may be replaced with reconstructedwatertight surfaces.

At 503, the “trim curves” may be updated with isocurves and knots forB-rep agreement.

Subsequent to completion of the postprocessing steps, a watertightCADfile comprising the postprocessed output surfaces may be output from thealgorithm.

Further Technical Algorithmic Detail

The following paragraphs present a more detailed and technicalexplanation of the steps involved in the watertightCAD method, accordingto some embodiments.

Traverse the Topology of the Boundary Representation to AccessSSI-Related Data

Detailed herein is further exposition on the preprocessing method forthe Post-SSI algorithm, 120, according to some embodiments.

In some embodiments, the Post-SSI algorithm may traverse the boundaryrepresentation to identify a set of topological entities associated witha surface-surface intersection, e.g., a pair of topological faces F₀ andF₁ that share a topological edge E. This identification may be performedin any of a variety of ways, e.g., depending on the data structureformat of the boundary representation. The face F₀ is associated with aspline surface S₀ and one or more parameter space trim curves whoseimages reside in the 2D parametric domain of the spline surface S₀. (Theboundary representation may store spline curves and spline surfaces interms of geometry data sets. For example, a geometry data set for acurve may include its knot vector and its curve control points. Thegeometry data set for a surface may include its knot vector pair and itssurface control points.) One of those parameter space trim curves, whichwe denote as C_(PS0), is associated with the edge E. Likewise, the faceF₁ is associated with a spline surface S₁ and one or more parameterspace trim curves whose images reside in the 2D parametric domain of thespline surface S₁. One of those parameter space trim curves, which wedenote as C_(PS1), is associated with the edge E. Furthermore, the edgeE is associated with a curve C_(MS) in model space.

As described below, the Post-SSI algorithm may operate on the surfacesplines S₀ and S₁ to respectively create a first set of one or moreoutput surface patches and a second set of one or more output surfacepatches, where a boundary of a union of images of the first set ofoutput surface patches and a boundary of a union of the images of thesecond set of output surface patches meet in a C⁰ continuous fashionalong the model space curve C_(MS). Thus, the gaps in the originalsurface-surface intersection will have been repaired.

The spline surfaces {S_(i)} may be tensor product spline surfaces ofdegree combination p_(i)×q_(j), where p_(i) and q_(i) are integersgreater than zero, where p_(i) denotes the degree in one parametricdimension, and q_(i) denotes the degree in the other parametricdimension. For example, in some embodiments p_(i)=q_(i)=3, i.e., thesurfaces are bicubic surfaces. In other embodiments, the surfaces may bebiquartic or biquintic surfaces, or have mixed degrees. In someembodiments, the surfaces may have different degree combinations. Forexample, one surface might be bicubic while another surface isbiquartic.

The object modeled by the boundary representation may include aplurality of topological face pairs that correspond to surface-surfaceintersections. For each such face pair, the steps detailed below may beperformed to construct sets of output surface patches that meet in a C⁰continuous fashion along their respective model space SSI curves. Thus,the Post-SSI algorithm may reconstruct all the original spline surfacesalong the surface-surface intersections in the model, making the modelready for any of various applications, without the need for humanintervention for gap repair.

The spline surface S_(i)=S_(i)(s^(i), t^(i)) is defined on a domainΩ^(i)=[s _(min) ^(i) ,s _(max) ^(i)]×[t _(min) ^(i) ,t _(max) ^(i)]⊂

²,and has knot vectors

^(i) and

^(i).Segmentation and Classification

The following paragraphs provide additional detail regarding theparameter space analysis steps for the Post-SSI algorithm, 220.

For each spline surface S_(i) involved in the surface-surfaceintersection, the corresponding parameter space trim curve C_(PSi)(i.e., the trim curve in the 2D parametric domain of the spline surfaceS_(i)) may be segmented. The segmentation process may partition theparameter space trim curve C_(PSi) into one or more segments so thateach segment can easily be classified as being either, “moreconveniently viewed as a function of the first surface parameter s^(i)”,or, “more conveniently viewed as a function of the second surfaceparameter t^(i)”. The segmentation process may be carried out usingcriteria that identify specific points on the parameter space trim curveC_(PSi). When operating on a neighborhood of a given segment, thesurface reconstruction process may replace the segment with either an{tilde over (s)}^(i) isocurve or a {tilde over (t)}^(i) isocurve withina new parametric surface domain, depending on the classification of thesegment.

Along with segmenting and classifying the parameter space trim curveC_(PSi), a check is made to identify points on the parameter space trimcurve known as extraordinary points (also known as singular points ordegenerate points of a tensor product surface patch edge).

Find Significant Points for the Parameter Space Trim Curve correspondingto Spline Surface

The following paragraphs provide additional detail regarding theparameter space analysis steps for the Post-SSI algorithm, 220.

Given the parameter space SSI curve C_(PSi)(u^(i))=[s^(i)(u^(i)),t^(i)(u^(i))]^(T) corresponding to the spline surface S_(i)(s^(i),t^(i)), identify significant points of a number of different types. (Thesuperscript i in the parameter u^(i) of the parameter space SSI curveC_(PSi) is a reminder that the trajectory of C_(PSi) resides in thedomain Ω^(i) of the spline surface S_(i).)

In some embodiments, there are four types of significant points, asfollows.

Type 0: The significant points of type 0 include the end points andinternal kinks and cusps of the curve C_(PSi). In other words, asignificant point of type 0 corresponds to a knot of valence greaterthan or equal to the degree p_(u) _(i) of the curve C_(PSi)(u^(i)).

Type 1: The significant points of type 1 include the inflection pointsof the curve C_(PSi), where an inflection point is a pointC_(PSi)(u^(i)) such that the slope relative to surface parameter s^(i)is equal to one or minus one:

${{\frac{\left( t^{i} \right)^{\prime}\left( u^{i} \right)}{\left( s^{i} \right)^{\prime}\left( u^{i} \right)}} = {{{{dt}^{i}/{ds}^{i}}} = 1}},$where (t^(i))′(u^(i)) denotes differentiation of t^(i) with respect tou^(i), and similarly, (s^(i))′(u^(i)) denotes differentiation of s^(i)with respect to u^(i).

Type 2: The significant points of type 2 include the stationary criticalpoints of the curve C_(PSi), i.e., points C_(PSi)(u^(i)) such that

$\frac{{dt}^{i}}{{du}^{i}} = 0.$In other words, a stationary critical point is a point where the curveC_(PSi) has zero slope, when viewed as a function of s^(i).

Type 3: The significant points of type 3 include the non-stationarycritical points of the curve C_(PSi), i.e., points C_(PSi)(u^(i)) suchthat

$\frac{{ds}^{i}}{{du}^{i}} = 0.$In other words, a non-stationary critical point is a point where thecurve C_(PSi) has infinite slope, when viewed as a function of s^(i).Special Cases in Significant Point Analysis

It should be observed that it is possible for the parameter space trimcurve C_(PSi) to have one or more line segments as part of itstrajectory. If the slope of such a line segment is a member of the set{0,1,−1, ∞}, that line segment would result in an infinite number ofsignificant points. Thus, such segments of special slope should beremoved from the curve C_(PSi) prior to the identification ofsignificant points. Methods to accomplish this identification andremoval of line segments from the trajectory of a curve are well knownin the art of curve processing.

Classification of Sections of Parameter Space Trim Curve C_(PSi)

The following paragraphs provide additional detail on 222 from FIG. 14,wherein segments between characteristic points found in 221 areidentified and classified.

Each significant point of the curve C_(PSi)=C_(PSi)(u^(i)) correspondsto a respective value of the curve parameter u^(i). The significantpoints of the curve C_(PSi) may be ordered according to the curveparameter u^(i). This ordering partitions the curve C_(PSi) intosegments. We use the index α to identify the significant pointsaccording to this ordering. Each segment is bounded in u^(i) by asuccessive pair (u_(α) ^(i), u_(α+1) ^(i)) of the significant points.

Each segment may be classified as being of type s^(i) or type t^(i)based on the types of its endpoints, e.g., as shown in FIG. 19. If thesegment is bounded by significant points C_(PSi)(u_(α) ^(i)) andC_(PSi)(u_(α+1) ^(i)), there are sixteen possible cases for the orderedpair(TypeC _(PSi)(u _(α) ^(i)),TypeC _(PSi)(u _(α+1) ^(i))),where Type C_(PS1)(u_(α) ^(i)) denotes the type of the significant pointC_(PSi)(u_(α) ^(i)), and Type C_(PSi)(u_(α+1) ^(i)) denotes the type ofthe significant point C_(PSi)(u_(α+1) ^(i)). As shown in FIG. 19, thetype of the segment is immediately resolved in certain cases of thosesixteen cases, which are labeled with s^(i) or t^(i). In other cases,the type of the segment must be checked as follows. Calculate thevelocity vector

$\frac{{dC}_{PSi}}{{du}^{i}} = \left\lbrack {\frac{{ds}^{i}}{{du}^{i}},\frac{{dt}^{i}}{{du}^{i}}} \right\rbrack^{T}$at any point in the interior of the segment (i.e., excluding the endpoints), and classify the segment based on:

${{Segment}\mspace{14mu}{Type}} = \left\{ {\begin{matrix}{{{type}\mspace{14mu} s^{i}},} & {{{if}\mspace{14mu}{{{dt}^{i}/{ds}^{i}}}} \leq 1} \\{{{type}\mspace{14mu} t^{i}},} & {{{if}\mspace{14mu}{{{dt}^{i}/{ds}^{i}}}} > 1}\end{matrix}.} \right.$Note that the choice of assigning the boundary case |dt^(i)/ds^(i)|=1 totype s^(i) is arbitrary. It could equally well have been assigned totype t^(i).

Furthermore, observe that the segment type classification is invariantunder exchange of the endpoint types. For example, the segment type ofpair (2,1) is the same as the segment type of pair (1,2). Finally, notethat the type pairs (3,2) and (2,3) are not possible if the segmentationhas been performed properly. (Each represents a transition between zeroslope and infinite slope, which will not occur on a continuous curvesegment without an intermediate significant point of type 1.) Thus, anerror warning may be issued if either of these type pairs isencountered.

Clustering Segments of Same Type into Intervals

In some embodiments, each maximal contiguous group of segments of thesame type may be combined to form a cluster (i.e., interval) of the sametype, e.g., as shown in FIG. 20. As another example, given a sequence ofsix segments having the corresponding sequence of types {s^(i), s^(i),s^(i), t^(i), t^(i), s^(i)}, then the first three segments may becombined to form a cluster of type s^(i); the next two segments may becombined to form a cluster of type t^(i); and the last segment wouldconstitute a singleton cluster of type s^(i).

In some embodiments, certain aspects of the processing to be describedbelow may be performed on the basis of a cluster.

Check for Extraordinary Point Locations at Ends of Loop Component Curvesand at Kinks/Cusps of the Parameter Space Curve

As part of the traversal of the boundary representation, the Post-SSIalgorithm may identify a loop or loops associated with each facecorresponding to spline surface S_(i) involved in the intersection. Theloop may be directed and include component curve(s), among which is theparameter space curve C_(PSi). Each of the component curve(s) maps froma corresponding 1D domain to the 2D parametric domain of the splinesurface S_(i). The loop partitions the 2D parametric domain intoregions. The orientation of the loop is typically used by the boundaryrepresentation as a mechanism to indicate which region(s) is to beretained. The other region(s) is discarded (i.e., counted as not beingpart of the modeled object).

For each successive pair of component curves (C_(LC(n)), C_(LC(n+1))) inthe loop, the point where the two component curves (of the pair) meetmay be tested for degeneracy as follows. (The notation LC(n) is meant tosuggest the n-th component curve of the loop. When the index value npoints to the last component curve of the loop, the index value n+1 isinterpreted as pointing to the first component curve of the loop.) Letvector vel_(a) denote the velocity vector of component curve C_(LC(n))at its end, and let vector vel_(b) denote the velocity vector ofcomponent curve C_(LC(n+1)) at its start. In other words, if we denotethe parameter of loop component C_(LC(n)) as r^(n),C_(LC(n))=C_(LC(n))(r^(n)), then

${{vel}_{a} = {{C_{{LC}{(n)}}^{(1)}\left( r_{end}^{n} \right)} = {\frac{{dC}_{{LC}{(n)}}}{{dr}^{n}}\left( r_{end}^{n} \right)}}},{{vel}_{b} = {{C_{{LC}{({n + 1})}}^{(1)}\left( r_{start}^{n + 1} \right)} = {\frac{{dC}_{{LC}{({n + 1})}}}{{dr}^{n + 1}}{\left( r_{start}^{n + 1} \right).}}}}$

Then the meeting point, C_(LC(n))(r^(end))=C_(LC(n+1))(r_(start)^(n+1)), is declared to be an extraordinary point if

${{135{^\circ}} \leq {\cos^{- 1}\left( \frac{{vel}_{a} \cdot {vel}_{b}}{{{vel}_{a}}{{vel}_{b}}} \right)} \leq {225{^\circ}}},$where vel_(a)·vel_(b) denotes the dot product of vectors vel_(a) andvel_(b), and ∥vel_(a)∥ and ∥vel_(b)∥ denote the Euclidean norm ofvectors vel_(a) and vel_(b), respectively.

If the loop arises as part of a non-manifold object, the looporientation may not be consistent with the above test criterion becauseof the particular definition employed for non-manifold curveorientations. The extraordinary point criterion may be realized indifferent ways, dependent on the specific implementation, but will notbe unrealizable because of these variations.

Test for Extraordinary Points at Kinks/Cusps.

Each kink and cusp along the parameter space curve C_(PSi) may be testedfor degeneracy. Let u_(ψ) ^(i) be the parametric location of a kink orcusp in the parameter space curve C_(PSi), and let vector vel_(a) denotethe derivative of the parameter space curve C_(PSi) evaluated atlocation u_(ψ) ^(i)−δ, and let vector vel_(b) denote the derivative ofthe parameter space curve C_(PSi) at location u_(ψ) ^(i)+δ, where δ is asmall positive real number.

${{vel}_{a} = {{C_{PSi}^{(1)}\left( {u_{\psi}^{i} - \delta} \right)} = {\frac{{dC}_{PSi}}{{du}^{i}}\left( {u_{\psi}^{i} - \delta} \right)}}},{{vel}_{b} = {{C_{PSi}^{(1)}\left( {u_{\psi}^{i} + \delta} \right)} = {\frac{{dC}_{PSi}}{{du}^{i}}{\left( {u_{\psi}^{i} + \delta} \right).}}}}$Then, the point C_(PSi)(u_(ψ) ^(i)) may be identified as anextraordinary point if vel_(a) and vel_(b) satisfy the above statedangular condition.Refinement Steps and Boundary Isocurve Reparameterization

In the following steps, the Post-SSI algorithm may refine the surfaces{S_(i)} and the model space curve C_(MS), i.e., apply knot refinement tothese entities. Each of these entities may be refined based on bothintrinsic location information and external location information. Whenrefining an entity based on location information that naturally belongsto that entity, the location information is referred to as intrinsiclocation information. For example, when refining a surface (or curve)based on its original knot values, the knot values are interpreted asintrinsic location information. As another example, when refining asurface based on the significant points residing within the parametricdomain of that surface, the significant points may be interpreted asintrinsic location information. Conversely, when refining an entitybased on location information that naturally belongs to another entity,the location information is referred to as external locationinformation. For example, when refining a surface based on thesignificant points residing in the parametric domain of another surface,the significant points are interpreted as external location information.As another example, when refining the model space curve C_(MS) based onthe knots of a surface, the knots are interpreted as external locationinformation.

Furthermore, as part of the geometry reconstruction process to bedescribed below, we may create two output tensor product surface patchessuch that (a) their boundaries meet continuously along a portion of themodel space curve and (b) the union of the images of the two outputsurface patches approximates a neighborhood in the image of the splinesurface S_(i), i.e., a neighborhood that straddles the surface-surfaceintersection. It would be desirable if isocurves in the first outputsurface patch could meet the respective isocurves of like parametricvalue in the second output surface patch, and if the point of meetingwere without parametric derivative discontinuity. (The component of thefirst derivative in the direction tangent to the model space curve ispreferably without discontinuity at the point of meeting.) In order toenforce these continuity conditions on the isocurves, isoparametricboundary curves in the complementary parameter direction may bereparametrized to achieve parametric agreement with the model spacecurve. These isoparametric boundary curves may be reparametrized in avariety of ways, e.g., depending on a number of features desired by theuser.

Refinement of Surfaces and Model Space Curve based on Intrinsic LocationInformation.

A) Refine the Model Space Curve by Knot Insertion at Knots of ModelSpace Curve.

The model space curve C_(MS)(v) may be refined by performing knotinsertion at each interior knot in the original knot vector V of modelspace curve C_(MS)(v), to raise the multiplicity of the knot to thedegree of the model space curve C_(MS). (Hereinafter, unless statedotherwise, when we refer to “refining” a given curve at a givenparametric location, we will mean the process of raising the knotmultiplicity at that parametric location to the degree of the givencurve. Hereinafter, unless stated otherwise, when we refer to “refining”a given surface at a given location in one of the surface's parametricvariables, we will mean the process of raising the knot multiplicity atthat location to the degree of the surface in that parametric variable.)As is well understood in the art of spline-based modeling, thisoperation causes the model space curve to be separated into intervals ofsimplified form (e.g., of Bézier form), without changing the trajectoryof the curve in space.

B) Refine Surface S_(i) by Knot Insertion at s^(i) Knots or t^(i) Knotsof Surface S_(i), Depending on Classification.

For a given segment (or cluster) of the parameter space curve C_(PSi),the spline surface S_(i) may be refined at certain interior surfaceknots of the spline surface S_(i), depending on the classification ofthe segment. Suppose x^(i)∈{s^(i), t^(i)} denotes the type of thesegment. The surface S_(i) may then be refined at interior x^(i) knotsof S_(i) that occur on the closed interval [x^(i)(u_(α) ^(i)),x^(i)(u_(α+1) ^(i))], where u_(α) ^(i) and u_(α+1) ^(i) denote theparameter values of C_(PSi)(u^(i)) that bound the segment. (An x^(i)knot of the surface S_(i) is either an s^(i) knot if the segment is oftype s^(i), or a t^(i) knot if the segment is of type t^(i).Furthermore, a y^(i) knot of the surface S_(i) is either a t^(i) knot ifthe segment is of type s^(i), or an s^(i) knot if the segment is of typet^(i).)

Recall that the parameter space curve C_(PSi)(u^(i)) has coordinatefunctions s^(i)(u^(i)) and t^(i)(u^(i)) such thatC _(PSi)(u ^(i))=[s ^(i)(u ^(i)),t ^(i)(u ^(i))]^(T).Thus, if the segment is of type x^(i)=s^(i), then[x ^(i)(u _(α) ^(i)),x ^(i)(u _(α+1) ^(i))]=[s ^(i)(u _(α) ^(i)),s^(i)(u _(α+1) ^(i))].Conversely, if the segment is type x^(i)=t^(i), then[x ^(i)(u _(α) ^(i)),x ^(i)(u _(α+1) ^(i))]=[t ^(i)(u _(α) ^(i)),t(u_(α+1) ^(i))].

C) Refine the Surface S_(i) based on the Significant Points of ParameterSpace Curve C_(PSi).

Given a segment of type x^(i)∈{s^(i), t^(i)} from the parameter spacecurve C_(PSi), the surface S_(i) may be refined at the x^(i)-coordinatelocations corresponding to the significant points that bound thesegment. In other words, the surface S_(i) may be refined at thelocations x^(i)(u_(α) ^(i)) and x^(i)(u_(α+1) ^(i)), where u_(α) ^(i)and u_(α+1) ^(i) denote the parameter values of curve C_(PSi)(u^(i))that bound the segment.

Refinement of Surfaces and Model Space Curve based on External LocationInformation.

A) Refine the Model Space Curve C_(MS) based on Surface Knots of EachSurface S_(i).

For a given segment (or cluster) of type x^(i)∈{s^(i), t^(i)} from theparameter space curve C_(PSi), the Post-SSI algorithm may refine themodel space curve C_(MS)(v) at locations corresponding to the interiorx^(i) knots of surface S_(i) that occur on the closed interval[x^(i)(u_(α) ^(i)), x (u_(α+1) ^(i))]. Furthermore, in some embodiments,any y^(i) knot of the surface S_(i) whose corresponding knot lineintersects the given segment (or cluster) of the parameter space curveC_(PSi) may be transferred to the model space curve C_(MS), and used torefine the model space curve C_(MS).

The above described refinements of the model space curve may beperformed for each segment of the parameter space curve C_(PSi) and/orfor each surface S_(i) involved in the surface-surface intersection.

An x^(i) knot of S_(i) may be mapped to the one-dimensional parametricdomain of the model space curve C_(MS)(v) in a variety of ways. Forexample, one may map a surface knot s_(ε) ^(i) onto the domain ofC_(MS)(v) via a minimization of distance between the t^(i) isocurveS_(i)(s_(ε) ^(i), t^(i)) and the model space curve C_(MS)(v)).Similarly, one may map a surface knot t_(η) ^(i) onto the domain ofC_(MS)(v) via a minimization of distance between the s^(i) isocurveS_(i)(s^(i), t_(η) ^(i)) and the model space curve C_(MS)(v)).

B) Refinement of the Model Space Curve based on Significant Points ofEach Parameter Space Curve C_(PSi).

For all values of the surface index i, the Post-SSI algorithm may refinethe model space curve C_(MS)(v) at locations {v_(∝),} corresponding tothe significant points {(s_(α) ^(i), t_(α) ^(i))} of the parameter spacecurve C_(PSi)(u_(α) ^(i)). There are a variety of ways to determine thelocations {v_(∝)}. For example, in one embodiment, one may map asignificant point (s_(α) ^(i), t_(α) ^(i)) of the parameter space curveC_(PSi) onto the 1D parametric domain of C_(MS)(v) via a minimization ofdistance between S_(i)(s_(α) ^(i), t_(α) ^(i)) and C_(MS)(v), and thenpoint inversion.

C) Refine Each Surface S_(i) Based on the Knots of the Model SpaceCurve.

Each surface S_(i) may be refined based on the interior knots of themodel space curve C_(MS). Given a knot v_(ψ) of the model space curveC_(MS), the knot v_(ψ) may be mapped to a point (s_(ψ) ^(i), t_(ψ) ^(i))in the 2D parametric domain of the surface S_(i). The point (s_(ψ) ^(i),t_(ψ) ^(i)) may then be mapped to a closest segment of the parameterspace curve C_(PSi). If the closest segment is of type x^(i)∈{s^(i),t^(i)}, the surface S_(i) may be refined at x_(ψ) ^(i). In other words,if the segment type x^(i)=s^(i), the surface S_(i) is refined ats^(i)=s_(ψ) ^(i); conversely, if the segment type x^(i)=t^(i), thesurface S_(i) is refined at t^(i)=t_(ψ) ^(i).

As noted above, a knot value v_(k) of the model space curve C_(MS) maybe mapped to a point (s_(ψ) ^(i), t_(ψ) ^(i)) in the 2D parametricdomain of the surface S_(i). There are a variety of ways to perform thismapping. In one embodiment, the knot value v_(ψ) may be mapped byminimizing the distance between C_(MS)(v_(ψ)) and S_(i)(s^(i), t^(i)),and then point inversion.

D) Refine the Surface S_(i) Based on External Significant Points.

The Post-SSI algorithm may refine the surface S_(i) based on externalsignificant points, i.e., significant points occurring in the 2Dparametric domain(s) of the other participating surface(s). For eachsignificant point (s_(α) ^(j), t_(α) ^(j)) of each other participatingsurface S_(i), j≠i, a corresponding point (x_(MAP) ^(i), t_(MAP) ^(i))in the (s^(i), t^(i)) domain of the surface S_(i) may be computed. Thepoint (s_(MAP) ^(i), t_(MAP) ^(i)) may be mapped to a closest segment ofthe parameter space curve C_(PSi). If the closest segment is of typex^(i)∈{s^(i), t^(i)}, the surface S_(i) may be refined at x^(i)=x_(MAP)^(i). In other words, if the segment type x^(i)=s^(i), the surface S_(i)may be refined at s^(i)=s_(MAP) ^(i); conversely, if the segment typex^(i)=t^(i), the surface S_(i) may be refined at t^(i)=t_(MAP) ^(i).

As noted above, a significant point (s_(α) ^(j), t_(α) ^(j)) of adifferent participating surface S_(j) may be mapped to a point (s_(MAP)^(i), t_(MAP) ^(i)) in the (s^(i), t^(i)) domain of the surface S_(i).This mapping may be performed in a variety of ways. For example, in oneembodiment, if the significant point (s_(α) ^(j), t_(α) ^(j)) hasalready been transferred to a value v_(α) in the domain of the modelspace curve C_(MS), the point (s_(MAP) ^(i), t_(MAP) ^(i)) may bedetermined by minimizing the distance between the point C_(MS)(v_(α))and the surface S_(i)(s^(i), t^(i)), and then point inversion. Inanother embodiment, the significant point (s_(α) ^(j), t_(α) ^(j)) maybe mapped more directly to the domain of the surface S_(i) by minimizingthe distance between the point S_(i)(s_(α) ^(j), t_(α) ^(j)) and thesurface S_(i)(s^(i), t^(i)), and then point inversion. This minimizationmay be interpreted as a projection from the surface S_(j) to the surfaceS_(i), or from the 2D parametric domain of surface S_(j) to the 2Dparametric domain of surface S_(i).

For the modeling of a 2-manifold, the trim boundaries of at most twotrimmed surfaces can meet along any given curve in model space. However,the principles of the present invention are not limited to the contextof 2-manifolds. A non-manifold structure may be created where the trimboundaries of more than two surfaces meet along the model space curve.Thus, in some embodiments, it is possible that there are more than twoparticipating surfaces {S_(i)}.

Selection of Domain for Surface Patch Extraction and Isocurve Extraction

The Post-SSI algorithm may organize the construction output geometryrelative to surface S_(i) based on successive segments (or clusters) ofthe parameter space curve C_(PSi). Within each segment (or cluster), thePost-SSI algorithm may organize the construction of output geometrybased on successive portions of the segment (or cluster). In particular,for each portion of the segment (or cluster), a corresponding domain forsurface patch reconstruction may be selected, herein referred to as theextraction domain. The extraction domain is a subdomain in the 2Dparametric domain Ω^(i) of the spline surface S_(i), and is so namedbecause a set of isocurves may be extracted from the restriction ofspline surface S_(i) to the extraction domain. The control points ofthese isocurves may be used to the construct output surface patch(es)corresponding to the portion of the parameter space curve C_(PSi). Theboundary of the extraction domain may be referred to as the extractionboundary.

For each surface S_(i), the extraction domains that correspondrespectively to the portions of the parameter space curve C_(PSi) mayform a partition of the 2D parametric domain of the surface S_(i), i.e.,the extraction domains may be non-overlapping regions and their unionmay be equal to the 2D parametric domain of the surface S_(i). Here theterm “non-overlapping” means that the intersection of differentextraction domains has zero area. It should be understood, however, thatadjacent extraction domains may meet along their boundaries. Forexample, the right boundary of one extraction domain may be the leftboundary of another extraction domain. Given a segment (or cluster) oftype x^(i)∈{s^(i), t^(i)} from the parameter space curve C_(PSi), thesegment (or cluster) may be partitioned into portions based on a set ofdivision points comprising:

-   -   the interior x^(i) knots of surface S_(i);    -   the x^(i) knots of surface S_(i) that have been derived from the        intrinsic and external significant points;    -   the x^(i) knots of surface S_(i) that have been derived from the        knots of the model space curve C_(MS).

The distinct division points may be ordered, and denoted as {x_(β)^(i)}, where β is an index according to the ordering. (The superscript iis a reminder that the division points are related to surface S_(i).However, we may drop the superscript when there is no need for thatreminder.) Each interval of the form [x_(β) ^(i), x_(β+1) ^(i)] definesa corresponding portion CP_(PSi,β) of the segment (or cluster), andgives rise to a corresponding extraction domain Ω_(β) ^(i). The portionCP_(PSi,β) is the restriction of the parameter space curve CP_(PSi) tothe interval [x_(β) ^(i), x_(β+1) ^(i)]. (The notation “CP” is meant tosuggest “Curve Portion”.) In some embodiments, the union of theextraction domains {Ω_(β) ^(i)} over the index may be equal to the 2Dparameteric domain Ω^(i) of the surface S_(i): Ω^(i)=∪_(β)Ω_(β) ^(i).

Given a portion CP_(PSi,β), we may define a corresponding rectangularextraction domain Ω_(β) ^(i) as follows. The x^(i) extent of theextraction domain Ω_(β) ^(i) may be set equal to the closed interval[x_(β) ^(i), x_(β+1) ^(i)]. (Note that we may refer to a portion withthe more simple notation CP_(PSi) when the dependency on β is notnecessary for the discussion at hand.) The extent of the extractiondomain Ω_(β) ^(i) in the complementary parameter y^(i)∈{s^(i), t^(i)},y^(i)≠x^(i), may be selected in various ways. If we denote the y^(i)extent of the extraction domain Ω_(β) ^(i) by the closed interval[y_(LB) ^(i), y_(UB) ^(i)] we haveΩ_(β) ^(i)=[xβ ^(i) ,x _(β+1) ^(i)]×[y _(LB) ^(i) ,y _(UB) ^(i)].(The notation “LB” and “UB” are meant to be suggestive of “lower bound”and “upper bound”.)

It may be desirable for the extraction domain Ω_(β) ^(i) to bound theselected portion CP_(PSi,β). The value y_(LB) ^(i) may be selected as avalue of parameter y^(i) that is strictly less than the minimum value ofthe parameter y^(i) achieved by the portion CP_(PSi,β), and the valuey_(UB) ^(i) may be selected as a value of parameter y^(i) that isstrictly greater than the maximum value of the parameter y^(i) achievedby the portion CP_(PSi,β). For example, the value y_(LB) ^(i) may beselected to be a y^(i) knot of the surface S_(i) that is strictly lessthan said minimum, and the value y_(UB) ^(i) may be selected to be ay^(i) knot of the surface S_(i) that is strictly greater than saidmaximum. As another example, the values y_(LB) ^(i) and y_(UB) ^(i) maybe selected so that the extraction domain Ω_(β) ^(i) spans the entirey^(i) extent of the domain Ω^(i) of the surface S_(i), i.e., y_(LB) ^(i)is set equal to the first y^(i) knot of the surface S_(i) and y_(UB)^(i) is set equal to the last y^(i) knot of the surface S_(i). As yetanother example, the values y_(LB) ^(i) and y_(UB) ^(i) may be selectedbased on user input.

The Post-SSI algorithm may extract a surface patch S_(i,β) from thespline surface S_(i), i.e., a surface patch corresponding to theextraction domain Ω_(β) ^(i). (The extraction may involve knot insertionat y_(LB) ^(i) and y_(UB) ^(i), if such insertion has not already beenperformed.) The image of the surface patch S_(i,β) in model space iscoincident with the image of the extraction domain Ω_(β) ^(i) under thespline map S_(i).

In some embodiments, previous steps in the Post-SSI algorithm will havesystematically exchanged refinement location information such as knotvalues and significant points between exchange participants (i.e., themodel space curve and surfaces). Thus, the above-described distinctdivision points {x_(β) ^(i)} along the x^(i) axis of the surface S_(i)will exist in one-to-one correspondence with division points {v_(β)} inthe 1D domain of the model space curve, and in one-to-one correspondencewith division points {x_(β) ^(j)} in each other surface S_(j), j≠i.Thus, the x^(i) extent of the extraction domain Ω_(β) ^(i) maycorrespond not only to the portion CP_(PSi,β) of the parameter spacecurve CP_(PSi), but also to a portion CP_(MS,β) of the model space curveC_(MS) and a portion CP_(PSj,β) of parameter space curve CP_(PSj) forall j≠i. One may interpret the above described process of exchange andrefinement as a common reduction to mutually correlated portions ofsimplified form.

Reparameterization of Isocurves on the Extraction Boundary.

The following paragraphs present additional detail regarding thereparameterization algorithm, 300.

The Post-SSI algorithm may reparametrize the x^(i) isoparametricboundary curves of the surface patch S_(i,β), e.g., to avoid theshearing of y^(i) isocurves of the reconstructed output patch(s) nearthe model space curve C_(MS). (In general, x^(i) isoparametric curvesmay be referred to herein as “transverse isoparametric curves”.Furthermore, y^(i) isoparametric curves may be referred to herein as“longitudinal isoparametric curves”.) In particular, the Post-SSIalgorithm may reparametrize the two x^(i) isocurves of the surface patchS_(i,β) corresponding respectively to the boundary values y_(LB) ^(i)and y_(UB) ^(i). We refer to these boundary isocurves as ISO_(y) _(LB)_(i) (x^(i)) and ISO_(y) _(UB) _(i) (x^(i)). Each of these boundaryisocurves may be reparametrized to achieve parametric agreement with themodel space curve. This parametric agreement may be described in termsof a map T_(i) and a projection P_(x) _(i) defined as follows.

The map T_(i) maps from the generic point C_(MS)(v) on the model spacecurve C_(MS) to the (s^(i), t^(i)) parametric domain of the surfaceS_(i). The map T_(i) may be realized, e.g., by distance minimization.The projection P_(x) _(i) maps the point (s^(i), t^(i)) onto theparameter x^(i). If the extraction domain Ω_(β) ^(i) is based on aportion CP_(PSi,β) of type x^(i)=s^(i), then P_(x) _(i) (s^(i),t^(i))=s^(i), i.e., the evaluation of P_(x) _(i) on the coordinate pair(s^(i), t^(i)) gives the value s^(i). Conversely, if the extractiondomain Ω_(β) ^(i) is based on a portion CP_(PSi,β) of type x^(i)=t^(i),then P_(x) _(i) (s^(i),t^(i))=t^(i), i.e., the evaluation of P_(x) _(i)on the coordinate pair (s^(i), t^(i)) gives the value t^(i).

We seek a reparameterization function x^(i)=ƒ_(x) _(i) (v) that mapsfrom the closed interval I_(β)=[v_(β), v_(β+1)] (or any other convenientinterval) onto the interval [x_(β) ^(i), x^(i) _(β+1)] such that, forall v in I_(β), the following condition is obeyed (or approximated):P _(x) _(i) T _(i)(C _(MS)(v))=ƒ_(x) _(i) (v).

(The expression “x^(i)=ƒ_(x) _(i) (v)” is interpreted to mean that theevaluation of function ƒ_(x) _(i) on the value v gives the value x^(i).)The reparameterization function ƒ_(x) _(i) may be linear, piecewiselinear or non-linear, and may be computed in any of various ways.

The two boundary isocurves ISO_(y) _(LB) _(i) (x^(i)) and ISO_(y) _(UB)_(i) (x^(i)) are reparametrized using the function ƒ_(x) _(i) to obtainreparametrized isocurves ISO_(y) _(LB) _(i) (v) and ISO_(y) _(UB) _(i)(v), respectively. For convenience, in the following steps we maycontinue to refer to the parameter x^(i), with the understanding that,in at least some embodiments, x^(i) agrees with parameter v, due to thepresent reparameterization.

Construction of Output Patch Geometry

The following paragraphs give additional algorithmic detail on thereconstruction steps 400 of the WatertightCAD method.

For each value of the surface index i, geometric data G_(i) derived fromthe surface patch S_(i,β) along with geometric data from the portionCP_(MS,β) of the model space curve C_(MS) may be used to construct up totwo output patches. The two output patches would correspond to oppositesides of the model space curve. In some embodiments, the output patchesare Bézier output patches, i.e., of Bézier form in both parametricdirections. In other embodiments, the output patches are of Bézier format least in the x^(i) direction, but may be of B-Spline or NURBS form inthe y^(i) direction. In yet other embodiments, the output patches may beof B-Spline, NURBS, or T-spline form in both the x^(i) and y^(i)directions, in which case, the model space curve need not be refined tothe level of Bézier portions.

The two output patches, denoted {tilde over (S)}_(iA,β) and {tilde over(S)}_(iB,β), may be defined respectively on rectangular domains {tildeover (Ω)}_(β) ^(iA) and {tilde over (Ω)}_(β) ^(iB). However, the outputpatches {tilde over (S)}_(iA,β) and {tilde over (S)}_(iB,β) may beassociated with two respective sub-domains of the extraction domainΩ_(β) ^(i) as follows. We may consider the extraction domain Ω_(β) ^(i)as being partitioned into two sub-domains Ω_(β) ^(iA) and Ω_(β) ^(iB)that meet along the distance-minimized pullback of the model space curveC_(MS) into the 2D parametric domain Ω^(i) of the surface S_(i). (Thegeneric point C_(MS)(v) on the model space curve C_(MS) could be mappedto a point in the 2D parametric domain of the surface S_(i) byminimizing the distance between S_(i)(s^(i), t^(i)) and the pointC_(MS)(v). This mapping may be referred to as a distance-minimizedpullback.) As described below, we may extract geometric data G_(iA)corresponding to the surface patch S_(i,β) and the sub-domain Ω_(β)^(iA), and extract geometric data G_(iB) corresponding to the surfacepatch S_(i,β) and the sub-domain Ω_(β) ^(iB). The output patch {tildeover (S)}_(iA,β) may be computed based on the geometric data G_(iA) andthe geometric data from the portion CP_(MS,β) of the model space curveC_(MS). The output patch {tilde over (S)}_(iB,β) may be computed basedon the geometric data G_(iB) and the geometric data from the portionCP_(MS,β).

For each value of the surface index i and for each k∈{A, B}, the outputpatch {tilde over (S)}_(ik,β) may have the desirable property that aboundary isocurve of that output patch {tilde over (S)}_(ik,β) will maponto the portion CP_(MS,β) of the model space curve C_(MS). Thus, theoutput patches {{tilde over (S)}_(ik,β): i∈{0,1}; k∈{A, B}} will allmeet each other along the portion CP_(MS,β) with C⁰ continuity. (Here weassume there are two surfaces participating in the surface-surfaceintersection. However, the principles of the present invention naturallygeneralize to any number of participating surfaces.)

Furthermore, if the above-described reparameterization step is performedfor each value of the surface index i, then for all v∈I_(β), i∈{0,1} andk∈{A, B}, the y^(i) isocurve of {tilde over (S)}_(ik,β) corresponding tothe value {tilde over (x)}^(i)=v will either start or end at the pointC_(MS)(v) on the model space curve. Thus, longitudinal isocurves indifferent output patches will meet at the same point on the model spacecurve if they correspond to the same coordinate value v. Because thisproperty holds for each portion CP_(MS,β) of the model space curve(i.e., each value of portion index β in the index set Φ), the parameterspaces of the output patches{{tilde over (S)} _(ik,β) :i∈{0,1},k∈{A,B},β∈Φ}are unified into a global parameter space.

(Here we again assume there are two surfaces participating in thesurface-surface intersection. However, the principles of the presentinvention naturally generalize to any number of participating surfaces.)

Yet further, the boundary isocurve of {tilde over (S)}_(ik,β) for theboundary remote from the portion CP_(MS,β) may have the same trajectoryin model space as the corresponding boundary isocurve of the surfacepatch S_(i,β).

The construction of each output patch {tilde over (S)}_(iA,β) and {tildeover (S)}_(iB,β) may involve the solution of a corresponding linearsystem of equations (or a corresponding collection of linear systems).In some embodiments, the coefficients of the linear system are known inadvance, and thus, the inverse matrix for this linear system may beprecomputed and hardcoded as part of a software and/or hardwareimplementation of the Post-SSI algorithm, for more efficient solution ofthe linear system.

In cases where the boundary representation is modeling a 2-manifoldobject, the Post-SSI algorithm may construct only one of the outputpatches {tilde over (S)}_(iA,β) and {tilde over (S)}_(iB,β) for eachvalue of the surface index i.

In cases where the boundary representation is modeling a non-manifoldobject, the Post-SSI algorithm may construct two or more output patchesthat meet along the model space curve with at least C⁰ continuity. Forexample, the Post-SSI algorithm may construct both the output patches{tilde over (S)}_(iA,β) and {tilde over (S)}_(iB,β).

In some embodiments, the two output patches {tilde over (S)}_(iA,β) and{tilde over (S)}_(iB,β) may be combined (by knot removal) along therespective boundaries that map to the model space curve, in order toform a single output patch {tilde over (S)}_(i,β), in which case the twoidentified boundaries become an interior isoparametric trim curve forthe single output patch {tilde over (S)}_(i,β).

In some embodiments, the above mentioned geometric data G_(i) derivedfrom the surface patch S_(i,β) may include:

-   -   the control points of one or both of the above-described        reparametrized transverse boundary isocurves;    -   the control points of boundary isocurves that extend in the        y^(i) direction; and    -   the control points of one or more non-boundary isocurves that        extend in the y^(i) direction.

The above mentioned geometric data from the portion CP_(MS,β) of themodel space curve C_(MS) may include the control points of that portion.

Sampling Longitudinal Isoparametric Curves from the Surface Patch

The Post-SSI algorithm may compute curve control points for a number ofy^(i) isocurves based on the surface control points of the surface patchS_(i,β). (By definition, a y^(i) isocurve corresponds to a fixed valueof x^(i) and extends in the y^(i) direction.) In particular, curvecontrol points for two boundary isocurves may be computed, i.e.,boundary isocurves corresponding respectively to the boundary valuesx^(i)=x_(β) ^(i) and x^(i)=x_(β+1) ^(i) of the extraction domain Ω_(β)^(i). Furthermore, curve control points for two or more non-boundaryisocurves may be computed. The two or more non-boundary isocurvescorrespond respectively to two or more distinct values of x^(i) in theinterior of the closed interval [x_(β) ^(i), x_(β+1) ^(i)]. In someembodiments, the number of the non-boundary isocurves may be equal toone less than the degree p_(x) _(i) of the surface patch S_(i,β) in thex^(i) direction (or the degree of the surface S_(i) in the x^(i)direction). For example, if p_(x) _(i) =3, there may be two non-boundaryisocurves. If p_(x) _(i) =2, there may be only one non-boundaryisocurve. The Greville abscissae may be desirable values of x^(i) orv^(i) to use for computing the p_(x) _(i) −1 non-boundary isocurves.Each of the non-boundary isocurves may correspond to a respective one ofthe Greville abscissae. The sampling of the non-boundary isocurves atthe Greville abscissae can be shown to produce a solution to the ensuinglinear system that is optimal in various metric norms.

More generally, if we denote the surface control net of the surfacepatch S_(i,β) by{P _(λ,ρ):0≤λ≤m _(x) _(i) ,0≤ρ≤n _(y) _(i) },with the index λ being associated with the parameter x^(i) and the indexρ being associated with the parameter y^(i), the number of thenon-boundary isocurves may be set equal to one less than m_(x) _(i) .

Let ISO_(x) _(λ) _(i) (y^(i)) denote the y^(i) isocurve of the surfacepatch S_(i,β) at x^(i)=x_(λ) ^(i):ISO_(x) _(λ) _(i) (y ^(i))=S _(i,β)(x _(λ) ^(i) ,y ^(i)).Techniques for computing the curve control points of isocurves given thesurface control points of a surface patch are well understood in the artof spline-based modeling.

In at least some embodiments, there may be m_(x) _(i) +1 longitudinalisocurves, corresponding respectively to the m_(x) _(i) +1 locations

x₀^(i) < x₁^(i) < … < x_(λ)^(i) < … < x_(m_(x^(i)) − 1)^(i) < x_(m_(x^(i)))^(i),with

x₀^(i) = x_(β)^(i)  and  x_(m_(x^(i)))^(i) = x_(β + 1)^(i).Restore Degree of Isoparametric Curves, as Needed

In some embodiments, the Post-SSI algorithm may employ a conventionalsoftware tool for computing the control points of an isocurve based onthe control points of a spline surface and a fixed value of a givensurface parameter. For some conventional tools, the isocurve returned bythe tool may be of lower degree than the degree of the surface in therelevant direction when the trajectory of the isocurve is simple enoughto admit an exact representation with a lower degree curve. (Forexample, imagine a bicubic spline surface generating by lofting, inwhich case the isocurves along one parametric direction may be linesegments.) This property of conventional tools, while useful in manycontexts, is not so useful in the present context. For the sake of thesurface reconstruction to be described below, we may require that theabove-mentioned y^(i) isocurves to be of degree equal to the degreep_(y) _(i) of the surface patch S_(i,β) in the y^(i) direction. To theextent that a software tool returns an isocurve of lower degree, thePost-SSI algorithm may elevate the degree of the isocurve to p_(y) _(i). Techniques for degree elevation are well known in the art ofspline-based modeling.

Divide the y^(i) Isoparametric Curves Based on Model Space Curve

Each of the above described y^(i) isocurves of the surface patch S_(i,β)may be divided into two sub-isocurves based on the model space curve. Insome embodiments, for each λ∈{0, 1, 2, . . . , m_(x) _(i) }, theisocurve ISO_(x) _(λ) _(i) (y^(i)) may be divided into two sub-isocurvesat a location y^(i)=y_(SSI,λ) ^(i) determined based on the model spacecurve (or the portion CP_(MS,β) of the model space curve). Inparticular, the Post-SSI algorithm may compute y_(SSI,λ) ^(i) as thevalue of the v_(λ) parameter that achieves the closest approach of theisocurve ISO_(x) _(λ) _(i) (y^(i)) to the model space curve C_(MS)(v).In other words, the Post-SSI algorithm may minimize the distance betweenISO_(x) _(λ) _(i) (y^(i)) and C_(MS)(v) over the space {(v, y^(i))}.(The value v^(k) that achieves the minimum distance may be used in theoptional constraint-based point repositioning described below.) Theisocurve ISO_(x) _(λ) _(i) (y^(i)) may be refined at y^(i)=y_(SSI,λ)^(i), to divide the isocurve into two sub-isocurves, denoted ISO_(x)_(λ) _(i) ^(A)(y^(i)) and ISO_(x) _(λ) _(i) ^(B)(y^(i)).

FIGS. 21A-21C show an example of the above-described isoparametriccurves sampling and division based on the model space curve. FIG. 21Ashows a spline surface S₀=S₀(s⁰, t⁰) intersected by spline surfaceS₁=S₁(s¹, t¹), a parameter space curve C_(PS0)(u⁰) corresponding tospline surface S₀, and a model space curve C_(MS)(v). FIG. 21B shows anexample of a reconstruction domain (in dark grey) corresponding to aportion of the parameter space curve C_(PS0)(u⁰) of type s⁰. (Thereconstruction domain is constrained from above by thedistance-minimized pullback of the model space curve, C_(MS)(v).) FIG.21C shows the corresponding sub-isoparametric curves{ISO_(x) _(λ) _(i) ^(A)(y ^(i) =t ⁰):x _(λ) ^(i) =s _(β) ₀ ⁰ ,s _(γ) ₀ ⁰,s _(γ) ₁ ⁰ ,s _(β) ₁ ⁰}generated from the above-described sampling and curve divisionprocedures.Reparameterization of y^(i) Isoparametric Subcurves to a Common KnotVector

A) Linear Reparameterization and Knot Cross-Seeding of the y^(i)Isoparametric Subcurves.

In some embodiments, it is possible that different sub-isocurves of theset {ISO_(x) _(λ) _(i) ^(A)(y^(i)): λ=0,1,2, . . . , m_(x) _(i) } havedifferent numbers of knots. (The sub-isocurves are typically ofdifferent lengths, and thus, a longer sub-isocurve might intercept alarger number of transverse knot lines of the surface patch S_(i,β) thana shorter sub-isocurve.) The Post-SSI algorithm may address this issueas follows. Each sub-isocurve ISO_(x) _(λ) _(i) ^(A)(y^(i)), λ∈{0, 1, 2,. . . , m_(x) _(i) }, may be linearly reparametrized to a commonparametric interval I_({tilde over (y)}) _(i) ^(iA)=[{tilde over(y)}_(min) ^(iA), {tilde over (y)}_(max) ^(iA)] in the y^(i) parameter,to obtain a corresponding linearly reparametrized sub-isocurveLRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)). (Thereare a wide variety of ways to select {tilde over (y)}_(min) ^(iA) and{tilde over (y)}_(max) ^(iA). For example, in one embodiment,I_({tilde over (y)}) _(i) ^(iA)=[0,1]. In another embodiment,I_({tilde over (y)}) _(i) ^(iA)=[y_(LB) ^(i), {tilde over (y)}_(SSI)^(i)], where {tilde over (y)}_(SSI) is strictly between y_(LB) ^(i) andy_(UB) ^(i), e.g., the midpoint of y_(LB) ^(i) and y_(UB) ^(i).)Furthermore, for each λ∈{0, 1, 2, . . . , m_(x) _(i) }, the Post-SSIalgorithm may identify any knots of the reparametrized sub-isocurveLRISO_({tilde over (x)}) _(μ) _(i) ^(A)({tilde over (y)}^(i)), μ∈{0, 1,2, . . . , m_(x) _(i) }, μ≠λ, that are not present in the knot vector ofthe reparametrized sub-isocurve LRISO_({tilde over (x)}) _(λ) _(i)^(A)({tilde over (y)}^(i)) and add those identified knots to the knotvector of LRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)).In other words, wherever one reparametrized sub-isocurve has a knot thatis absent from the knot vector of a different reparametrizedsub-isocurve, we may add that knot to the knot vector of the differentreparametrized sub-isocurve. As a result, each of the reparametrizedsub-isocurves will share a common knot vector.

FIG. 21D shows an example of the linear reparameterization based on thesub-isocurves shown in FIG. 21C.

B) Non-Linear Reparameterization of the y^(i) Isoparametric Subcurves.

In some embodiments, each sub-isocurve ISO_(x) _(λ) _(i) ^(A)(y^(i)),λ∈{0, 1, 2, . . . , m_(x) _(i) }, may be non-linearly reparametrizedwith a corresponding reparameterization function y^(i)=ƒ_(x) _(k) _(i)^(A)({tilde over (y)}^(i)), to obtain a corresponding reparametrizedsub-isocurve NLRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over(y)}^(i)), so that the reparametrized sub-isocurves{NLRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i))} aredefined on a common parametric interval I_({tilde over (y)}) _(i)^(iA)=[{tilde over (y)}_(min) ^(iA), {tilde over (y)}_(max) ^(iA)] andhave a common knot vector. For example, the functions {ƒ_(x) _(λ) _(i)^(A)} may be 1D spline curves of degree p_(Reparam), in which case thesub-isocurves {NLRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over(y)}_(i))} will be of degree p_(y) _(i) p_(REPARAM), where p_(y) _(i) isthe degree of the sub-isocurves {ISO_(x) _(λ) _(i) ^(A)(y^(i))}.(Non-linear reparameterization implies p_(REPARAM) is greater than one.)

In an alternative embodiment, the longest of the sub-isocurves and theremaining sub-isocurves may be addressed differently, but with the samegoal of obtaining sub-isocurves defined on a common knot vector. Inparticular, one may first identify the longest of the sub-isocurves. Letx_(LONG) ^(iA) denote the value of x_(λ) ^(i) corresponding to thelongest sub-isocurve. Each remaining sub-isocurve ISO_(x) _(λ) _(i)^(A)(y^(i)), x_(λ) ^(i)≠x_(LONG) ^(iA), may then be non-linearlyreparametrized with a corresponding reparameterization functiony^(i)=ƒ_(x) _(λ) _(i) ^(A)({tilde over (y)}^(i)) of common degreep_(REPARAM), to obtain a corresponding non-linearly reparametrizedsub-isocurve, denoted EVRIS0_({tilde over (x)}) _(λ) _(i) ^(A)({tildeover (y)}^(i)), of degree p_(y) _(i) p_(Reparam). The longestsub-isocurve may then be degree elevated to degree p_(y) _(i)p_(Reparam), to obtain a degree-elevated sub-isocurve, denotedEVRIS0_({tilde over (x)}) _(LONG) _(iA) ^(A)({tilde over (y)}^(i)). Thenonlinear reparameterizations and the degree elevation are performed insuch a fashion that the resulting set of m_({tilde over (x)}) _(i) +1sub-isocurves {EVRIS0_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over(y)}^(i)): λ∈{0, 1, 2, . . . , m_({tilde over (x)}) _(i) }} are ofdegree p_(y) _(i) p_(Reparam) and share a common knot vector. (Theprefix “EV” is meant as a reminder of the elevation of the longestsub-isocurve.) For example, if p_(y) _(i) =3, p_(Reparam)=3, and thesub-isocurve

ISO_(x_(m_(x^(i)))^(i))^(A)(y^(i))is the longest of the sub-isocurves, then each of the sub-isocurvesISO_(x) _(λ) _(i) ^(A)(y^(i)), λ∈{1, 2, . . . , m_(x) _(i) } may benon-linearly reparametrized with a corresponding degree-3reparameterization function y^(i)=ƒ_(x) _(λ) _(i) ^(A)({tilde over(y)}_(i)), and the sub-isocurve

ISO_(x_(m_(x^(i)))^(i))^(A)(y^(i))may be degree elevated to degree 9, to obtain a common knot vector. FIG.21E shows an example of the above discussed non-linearreparameterization scheme, using the sub-isocurves of FIG. 21C.

In the following discussion of geometry reconstruction, it may not besignificant how the sub-isocurves {ISO_(x) _(λ) _(i) ^(A)(y^(i))} havebeen reparametrized to a common knot vector. Thus, we may drop theprefix “L”, “NL” and “EV”, and refer simply to reparametrizedsub-isocurves {RISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over(y)}^(i))}. This notation is meant to encompass any type ofreparameterization being applied to the sub-isocurves to obtainreparametrized sub-isocurves that share a common knot vector.

C) Selection of Domain for Construction of Output Patch {tilde over(S)}_(iA,β).

At this point, we may select the domain {tilde over (Ω)}_(β) ^(iA) forthe output patch {tilde over (S)}_(iA,β). This domain may be interpretedas a reconstruction domain because the output patch {tilde over(S)}_(iA,β) may be constructed so as to rebuild (or modify) the geometryof the surface patch S_(i,β) on the extraction sub-domain Ω_(β) ^(iA).

In some embodiments, the reconstruction domain {tilde over (Ω)}_(β)^(iA) may be a subregion of the extraction domain Ω_(β) ^(i). In someembodiments, the reconstruction domain {tilde over (Ω)}_(β) ^(iA) may beset equal to the rectangle[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1) ^(i)]×[{tilde over(y)} _(min) ^(iA) ,{tilde over (y)} _(max) ^(iA)],where {tilde over (y)}_(min) ^(iA) and {tilde over (y)}_(max) ^(iA) arerespectively the first and last knot values of the common knot vector ofthe reparametrized sub-isocurves {RISO_({tilde over (x)}) _(λ) _(i)^(A)({tilde over (y)}^(i))}, i.e., the minimum and maximum values of thecommon interval on which the reparametrized sub-isocurves are defined.

In other embodiments, the reconstruction domain {tilde over (Ω)}_(β)^(iA) may be set equal to the rectangle[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1) ^(i)]×[{tilde over(y)} _(LIK) ^(iA) ,{tilde over (y)} _(max) ^(iA)],where {tilde over (y)}_(max) ^(iA) is the last knot value of the commonknot vector, and {tilde over (y)}_(LIK) ^(iA) is the last interior knotof the common knot vector. (An interior knot of a curve is a knotoccurring in the interior of the domain of that curve.) Eachreparametrized sub-isocurve RISO_({tilde over (x)}) _(λ) _(i)^(A)({tilde over (y)}^(i)), λ∈{0, 1, 2, . . . , m_({tilde over (x)})_(i) }, may be refined at {tilde over (y)}^(i)={tilde over (y)}_(LIK)^(A) (to obtain a corresponding Bézier sub-isocurveBISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)) on thedomain [{tilde over (y)}_(LIK) ^(iA), {tilde over (y)}_(max) ^(iA)].

D) Counterpart Processing of Sub-Isocurves on Other Side of Model SpaceCurve.

Note that similar processing operations may be applied to thesub-isocurves on the other side of the model space curve, i.e., on thesub-isocurves {ISO_(x) _(λ) _(i) ^(B)(y^(i)): λ=0,1,2, . . . , m_(x)_(i) }.

In some embodiments, each sub-isocurve ISO_(x) _(λ) _(i) ^(B)(y^(i)),λ∈{0, 1, 2, . . . , m_(x) _(i) }, may be linearly reparametrized to acommon parametric interval I_({tilde over (y)}) _(i) ^(iB)=[{tilde over(y)}_(min) ^(iB), {tilde over (y)}_(max) ^(iB)] in the y^(i) parameter,to obtain a corresponding reparametrized sub-isocurveLRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}^(i)). (Thereare a wide variety of ways to select {tilde over (y)}_(min) ^(iB) and{tilde over (y)}_(max) ^(iB). For example, in one embodiment,I_({tilde over (y)}) _(i) ^(iB)[0,1]. In another embodiment,I_({tilde over (y)}) _(i) ^(iB)=[{tilde over (y)}_(SSI) ^(i), {tildeover (y)}_(UB) ^(i)], where {tilde over (y)}_(LB) ^(i)<{tilde over(y)}_(SSI) ^(i)<{tilde over (y)}_(UB) ^(i).) Furthermore, for each λ∈{0,1, 2, . . . , m_({tilde over (x)}) _(i) }, the Post-SSI algorithm mayidentify any knots of the reparametrized sub-isocurveLRISO_({tilde over (x)}) _(μ) _(i) ^(B)({tilde over (y)}^(i)), μ∈{0, 1,2, . . . , m_({tilde over (x)}) _(i) }, μ≠λ, that are not present in theknot vector of the reparametrized sub-isocurve LRISO_({tilde over (x)})_(λ) _(i) ^(B)({tilde over (y)}^(i)), and add those identified knots tothe knot vector of LRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over(y)}^(i)). As a result, each of the reparametrized sub-isocurves willshare a common knot vector.

In some embodiments, each sub-isocurve ISO_({tilde over (x)}) _(λ) _(i)^(B)({tilde over (y)}^(i)), λ∈{0, 1, 2, . . . , m_(x)i}, may benon-linearly reparametrized with a corresponding reparameterizationfunction y^(i)=ƒ_(x) _(λ) _(i) ^(B)({tilde over (y)}^(i)), to obtain acorresponding reparametrized sub-isocurve NLRISO_({tilde over (x)}) _(λ)_(i) ^(B)({tilde over (y)}^(i)), so that the reparametrizedsub-isocurves {NLRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over(y)}^(i))} are defined on a common parametric intervalI_({tilde over (y)}) _(i) ^(iB)=[{tilde over (y)}_(min) ^(iB), {tildeover (y)}_(max) ^(iB)] and have a common knot vector. For example, thefunctions {ƒ_(x) _(λ) _(i) ^(B)} may be 1D spline curves of degreep_(Reparam), in which case the sub-isocurves {NLRISO_({tilde over (x)})_(λ) _(i) ^(B)({tilde over (y)}^(i))} will be of degree p_(y) _(i)p_(REPARAM), where p_(y) _(i) is the degree of the sub-isocurves{ISO_(x) _(λ) _(i) ^(B)(y^(i))}.

In an alternative embodiment, the longest of the sub-isocurves and theremaining sub-isocurves may be addressed differently, but with the samegoal of obtaining sub-isocurves defined on a common knot vector. Inparticular, one may first identify the longest of the sub-isocurves Letx_(LONG) ^(iB) denote the value of x_(λ) ^(i) corresponding to thelongest sub-isocurve. Each remaining sub-isocurve ISO_(x) _(λ) _(i)^(B)({tilde over (y)}^(i)), x_(λ) ^(i)≠x_(LONG) ^(iB), may then benon-linearly reparametrized with a corresponding reparameterizationfunction y_(λ) ^(i)=ƒ_(x) _(λ) _(i) ^(B)({tilde over (y)}^(i)) of commondegree p_(Reparam), to obtain a corresponding non-linearlyreparametrized sub-isocurve, denoted EVRISO_({tilde over (x)}) _(λ) _(i)^(B)({tilde over (y)}^(i)) of degree p_(i) _(i) p_(REPARAM). The longestsub-isocurve may then be degree elevated to degree p_(y) _(i)p_(REPARAM), to obtain a degree-elevated sub-isocurveEVRISO_({tilde over (x)}) _(LONG) _(iB) ^(B)({tilde over (y)}^(i)). Thenonlinear reparameterizations and the degree elevation are performed insuch a fashion that the resulting set of m_({tilde over (x)}) _(i) +1sub-isocurves {EVRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over(y)}^(i)): λ∈{0, 1, 2, . . . , m_({tilde over (x)}) _(i) }} are ofdegree p_(y) _(i) p_(REPARAM) and share a common knot vector.

In the following discussion of geometry reconstruction, it may not besignificant how the sub-isocurves {ISO_(x) _(λ) _(i) ^(B)(y^(i))} havebeen reparametrized to a common knot vector. Thus, we may drop theprefix “L”, “NL” and “EV”, and refer simply to reparametrizedsub-isocurves {RISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over(y)}^(i))}. This notation is meant to encompass any type ofreparameterization being applied to the sub-isocurves to obtainreparametrized sub-isocurves that share a common knot vector.

At this point, we may select the domain {tilde over (Ω)}_(β) ^(iB) forthe output patch {tilde over (S)}_(iB,β). This domain may be interpretedas a reconstruction domain because the output patch {tilde over(S)}_(iB,β) may be constructed so as to rebuild (or modify) the geometryof the surface patch S_(i,β) on the extraction sub-domain Ω_(β) ^(iB).

In some embodiments, the reconstruction domain {tilde over (Ω)}_(β)^(iB) may be a subregion of the extraction domain Ω_(β) ^(i). In someembodiments, the reconstruction domain may be set equal to the rectangle[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1) ^(i)]×[{tilde over(y)} _(min) ^(iB) ,{tilde over (y)} _(max) ^(iB)],where {tilde over (y)}_(min) ^(iB) and {tilde over (y)}_(max) ^(iB) arerespectively the first and last knot values of the common knot vector ofthe reparametrized sub-isocurves {RISO_({tilde over (x)}) _(λ) _(i)^(B)({tilde over (y)}^(i))}, i.e., the minimum and maximum values of thecommon interval on which the reparametrized sub-isocurves are defined.

In other embodiments, the reconstruction domain {tilde over (Ω)}_(β)^(iB) may be set equal to the rectangle[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1) ^(i)]×[{tilde over(y)} _(min) ^(iB) ,{tilde over (y)} _(FIK) ^(iB)],where {tilde over (y)}_(min) ^(iB) is the first knot value of the commonknot vector, and {tilde over (y)}_(FIK) ^(iB) is the first interior knotof the common knot vector. Each reparametrized sub-isocurveRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}^(i)), λ∈{0, 1,2, . . . , m_({tilde over (x)}) _(i) }, may be refined at {tilde over(y)}^(i)={tilde over (y)}_(FIK) ^(iB) to obtain a corresponding Béziersub-isocurve BISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over(y)}^(i)) on the domain [{tilde over (y)}_(min) ^(iB), {tilde over(y)}_(FIK) ^(iB)].Optional: Constraint-Based Repositioning of the Control Points of theIsoparametric Subcurves

In some embodiments, for each λ∈{0,1, 2, . . . , m_(x) _(i) }, thePost-SSI algorithm may perform constraint-based point repositioning onthe controls points of the sub-isocurve (RISO_({tilde over (x)}) _(λ)_(i) ^(A)({tilde over (y)}^(i)) and the control points of thecounterpart sub-isocurve RISO_({tilde over (x)}) _(λ) _(i) ^(B)({tildeover (y)}^(i)) so that those sub-isocurves meet on the model space curveC_(MS) and achieve a specified degree of continuity at the meeting pointon the model space curve C_(MS). (The desired degree of continuity maybe specified by a user.)

Let {{tilde over (Q)}_(λ,ρ) ^(A): 0≤λ≤n_({tilde over (y)}) _(i) ^(A)}denote the set of control points of the sub-isocurveRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)). (Whilethis set of control points depends among other things on surface indexi, we have not added that index to the control point notation, forsimplicity of discussion.) Among the two end control points of thesub-isocurve RISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over(y)}^(i)) i.e., {tilde over (Q)}_(λ,0) ^(A) and

${\overset{\sim}{Q}}_{\lambda,n_{{\overset{\sim}{y}}^{i}}^{A}}^{A},$note that

${\overset{\sim}{Q}}_{\lambda,n_{{\overset{\sim}{y}}^{i}}^{A}}^{A}$is closest to the model space curve C_(MS).

Let {{tilde over (Q)}_(λ,ρ) ^(B): 0≤λ≤n_({tilde over (y)}) _(i) ^(B)}denote the set of control points of the sub-isocurveRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}^(i)). (As above,we suppress the dependency on surface index, for ease of discussion.)Among the two end control points of the sub-isocurveRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}_(i)) i.e.,{tilde over (Q)}_(λ,ρ) ^(B) and

${\overset{\sim}{Q}}_{\lambda,n_{{\overset{\sim}{y}}^{i}}^{B}}^{B},$note that

${\overset{\sim}{Q}}_{\lambda,0}^{B}$is closest to the model space curve C_(MS). (Because the sub-isocurvesare constructed from a surface patch of the original surface S_(i), thesub-isocurves generically miss the model space curve.)

To guarantee that the sub-isocurves RISO_({tilde over (x)}) _(λ) _(i)^(A)({tilde over (y)}_(i)) and RISO_({tilde over (x)}) _(λ) _(i)^(B)({tilde over (y)}^(i)) meet on the model space curve C_(MS), oneneed only set both

${\overset{\sim}{Q}}_{\lambda,n_{{\overset{\sim}{y}}^{i}}^{A}}^{A}$and

${\overset{\sim}{Q}}_{\lambda,0}^{B}$equal to a selected point on the model space curve, e.g., the pointC_(MS)(v^(k)) determined above:

${\overset{\sim}{Q}}_{\lambda,n_{{\overset{\sim}{y}}^{i}}^{A}}^{A} = {{\overset{\sim}{Q}}_{\lambda,0}^{B} = {{C_{MS}\left( v^{k} \right)}.}}$

To achieve C¹ continuity between the sub-isocurvesRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)) andRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}^(i)), inaddition to the constraint of meeting on the model space curve, onewould reposition the control points

${\overset{\sim}{Q}}_{\lambda,{({n_{{\overset{\sim}{y}}^{i}}^{A} - 1})}}^{A},$and

${\overset{\sim}{Q}}_{\lambda,1}^{B}$to match the derivatives

$\frac{{dRISO}_{{\overset{\sim}{x}}_{\lambda}^{i}}^{A}\left( {\overset{\sim}{y}}^{i} \right)}{dy}\mspace{14mu}{and}\mspace{14mu}\frac{{{dRISO}_{{\overset{\sim}{x}}_{\lambda}^{i}}^{B}\left( {\overset{\sim}{y}}^{i} \right)},}{dy}$at the point of meeting. Furthermore, the repositioning of those pointsmay be constrained so that the derivative vector at the point of meetingon the model space curve equals (within a specified tolerance) thederivative vector of the original sub-isocurves at their original pointof meeting on surface S_(i). (Because the original sub-isocurves aregenerated by dividing a larger isocurve, they have the same derivativesat their point of meeting on the surface S_(i).)

To achieve C² continuity between the sub-isocurvesRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)) andRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}^(i)), inaddition to the constraint of meeting on the model space curve, onewould reposition the control points

${\overset{\sim}{Q}}_{\lambda,{({n_{{\overset{\sim}{y}}^{i}}^{A} - 2})}}^{A},$

${\overset{\sim}{Q}}_{\lambda,{({n_{{\overset{\sim}{y}}^{i}}^{A} - 1})}}^{A},${tilde over (Q)}_(λ,1) ^(B) and {tilde over (Q)}_(λ,2) ^(B).Furthermore, the repositioning of those points may be constrained sothat the first and second derivatives at the point of meeting on themodel space curve are equal (within a specified tolerance) to therespective derivatives of the original sub-isocurves at their originalpoint of meeting on surface S_(i).

More generally, to achieve C^(n) continuity between the sub-isocurvesRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)) andRISO_({tilde over (x)}) _(λ) _(i) ^(B)({tilde over (y)}^(i)), inaddition to the constraint of meeting on the model space curve, onewould reposition the control points

$\left\{ {{\overset{\sim}{Q}}_{\lambda,{({n_{{\overset{\sim}{y}}^{i}}^{A} - n})}}^{A},\ldots\mspace{14mu},{\overset{\sim}{Q}}_{\lambda,{({n_{{\overset{\sim}{y}}^{i}}^{A} - 2})}}^{A},{\overset{\sim}{Q}}_{\lambda,{({n_{{\overset{\sim}{y}}^{i}}^{A} - 1})}}^{A},{\overset{\sim}{Q}}_{\lambda,1}^{B},{\overset{\sim}{Q}}_{\lambda,2}^{B},\ldots\mspace{14mu},{\overset{\sim}{Q}}_{\lambda,n}^{B}} \right\}.$Furthermore, the repositioning of those points may be constrained sothat the first L derivatives at the point of meeting on the model spacecurve are equal (within a specified tolerance) to the respectivederivatives of the original sub-isocurves at their original point ofmeeting on surface S_(i). Techniques for performing constraint-basedrepositioning of curve control points on general spline curves are wellunderstood and need not be explained to those of ordinary skill in theart of spline theory.

In some embodiments, the above described constraint-based pointrepositioning may be performed on the Bézier sub-isocurves{BISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i))} and/orthe Bézier sub-isocurves {BISO_({tilde over (x)}) _(λ) _(i) ^(B)({tildeover (y)}^(i))}.

Construction of Output Patch and/or Output Patch {tilde over(S)}_(iB,β).

The Post-SSI algorithm may construct the surface control points for theoutput patch {tilde over (S)}_(iA,β) based on a set of isoparametriccurve data including:

-   -   (a) the control points of the portion CP_(MS,β) of the model        space curve, which may be interpreted as the control points for        a boundary isocurve of the output patch {tilde over (S)}_(iA,β);        and    -   (b) the control points of the y^(i) sub-isocurves        {RISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}        ^(i)):λ∈{1,2, . . . ,m _({tilde over (x)}) _(i) }},    -   or a subset of those control points.        See FIGS. 22A-22C and FIGS. 23A-23E to accompany the definitions        below.

In some embodiments, the set of isoparametric curve data may alsoinclude the control points of the boundary isocurve ISO_(y) _(LB) _(i)(x^(i)). In some embodiments, the set of isoparametric curve data mayinclude the interior control points of the y^(i) sub-isocurves{RISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)): k∈{0, 1,2, . . . , m_({tilde over (x)}) _(i) }}, but exclude the end controlpoints of those sub-isocurves. A control point of a spline curve is saidto be an interior control point if it is not an end control point ofthat spline curve.

The output patch {tilde over (S)}_(iA,β) may be defined on the abovedescribed reconstruction domain {tilde over (Ω)}_(β) ^(iA). Forspecificity of discussion, suppose the reconstruction domain {tilde over(Ω)}_(β) ^(iA) is given by:[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1) ^(i)]×[{tilde over(y)} _(LB) ^(i) ,{tilde over (y)} _(SSI) ^(i)],where {tilde over (y)}_(LB) ^(i)<{tilde over (y)}_(SSI) ^(i)<{tilde over(y)}_(UB) ^(i), where {tilde over (y)}_(LB) ^(i) and {tilde over(y)}_(UB) ^(i) are respectively the lower and upper bounds of theextraction domain Ω_(β) ^(i).

Let {{tilde over (P)}_(θ,ξ) ^(A): 0≤θ≤m_({tilde over (x)}) _(i) ,0≤ξ≤n_({tilde over (y)}) _(i) ^(A)} denote the surface control net ofthe output patch {tilde over (S)}_(iA,β), where n_({tilde over (y)})_(i) ^(A)+1 is the number of curve control points in each sub-isocurveRISO_({tilde over (x)}) _(λ) _(i) ^(A)({tilde over (y)}^(i)). In thespecial case where the sub-isocurves are Bézier sub-isocurves,n_({tilde over (y)}) _(i) ^(A)=p_({tilde over (y)}) _(i) , wherep_({tilde over (y)}) _(i) is the degree of the Bézier sub-isocurves.Furthermore, in the special case where the surface patch {tilde over(S)}_(i,β) is of Bézier form in the {tilde over (x)}^(i) direction,m_({tilde over (x)}) _(i) =p_({tilde over (x)}) _(i) , wherep_({tilde over (x)}) _(i) is the degree in the {tilde over (x)}^(i)direction.

In the rest of this section, for ease of discussion, we will drop thesurface index i from the parameters {tilde over (x)}^(i) and {tilde over(y)}^(i), the limit values m_({tilde over (x)}) _(i) andn_({tilde over (y)}) _(i) ^(A), the sample location {tilde over (x)}_(λ)^(i), and the degrees p_({tilde over (x)}) _(i) and p_({tilde over (y)})_(i) .

We want the surface control points of the output patch {tilde over(S)}_(iA,β) to be computed so that the portion CP_(MS,β) of the modelspace curve C_(MS) is a boundary isocurve of the output patch {tildeover (S)}_(iA,β), i.e., the boundary isocurve along the boundarycorresponding to {tilde over (y)}={tilde over (y)}_(SSI). This conditionmay be achieved by setting the boundary row{{tilde over (P)} _(θ,n) _({tilde over (y)}) _(A) ^(A):0≤θ≤m_({tilde over (x)})}of surface control points for the output patch equal to the controlpoints of the portion CP_(MS,β). (It is a mathematical fact that thecurve control points of a boundary isocurve of a tensor product splineare respectively identical to the surface control points of the tensorproduct spline along the same boundary.)

Similarly, in some embodiments, we may want the surface control pointsof the output patch S_(iA,β) to be computed so that the boundaryisocurve ISO_(y) _(LB) (x) of S_(i,β) is a boundary isocurve of theoutput patch S_(iA,β) along the boundary {tilde over (y)}={tilde over(y)}_(LB). This condition may be achieved by setting the boundary row

$\left\{ {{\overset{\sim}{P}}_{\theta,0}^{A}:{0 \leq \theta \leq m_{\overset{\sim}{x}}}} \right\}$of surface control points equal to the control points of the boundaryisocurve ISO_(y) _(LB) (x).

For the boundary column

$\left\{ {{\overset{\sim}{P}}_{0,\xi}^{A}:{0 \leq \xi \leq n_{\overset{\sim}{y}}^{A}}} \right\}$of surface control points of the output patch {tilde over (S)}_(iA,β),we may set the interior members of that column equal to thecorresponding interior control points of the boundary sub-isocurveRISO_({tilde over (x)}) ₀ ^(A)({tilde over (y)}^(i)). (Recall that{tilde over (x)}₀={tilde over (x)}_(β).) The end members of this columnare already addressed by the above described control pointidentifications.

For the boundary column

$\left\{ {{\overset{\sim}{P}}_{m_{\overset{\sim}{x}},\xi}^{A}:{0 \leq \xi \leq n_{\overset{\sim}{y}}^{A}}} \right\}$of surface control points of the output patch {tilde over (S)}_(iA,β),we may set the interior members of that column equal to thecorresponding interior control points of the boundary sub-isocurve

${{RISO}_{{\overset{\sim}{x}}_{m_{\overset{\sim}{x}}}}^{A}\left( \overset{\sim}{y} \right)}.$(Recall that {tilde over (x)}_(m) _({tilde over (x)}) ={tilde over(x)}_(β+1).) The end members of this column are already addressed by theabove described control point identifications.

Thus far we have accounted for all the boundary control points of thesurface control net, leaving only the(m_({tilde over (x)})−1)(n_({tilde over (y)}) ^(A)−1) interior surfacecontrol points:

$\left\{ {{{\overset{\sim}{P}}_{\theta,\xi}^{A}:{0 \leq \theta \leq m_{\overset{\sim}{x}}}},{0 \leq \xi \leq n_{\overset{\sim}{y}}^{A}}} \right\}$to be determined. The interior surface control points may be computed bysolving a linear system based on the fundamental equation for computingthe curve control points of an isocurve based on the surface controlpoints of a tensor product spline. In the present context, note that the{tilde over (x)}={tilde over (x)}_(λ) isocurve of the output patch{tilde over (S)}_(iA,β) is given by:

$\begin{matrix}{{{\overset{\sim}{S}}_{{iA},\beta}\left( {{\overset{\sim}{x}}_{\lambda},\overset{\sim}{y}} \right)} = {\sum\limits_{\theta = 0}^{m_{\overset{\sim}{x}}}\;{\sum\limits_{\xi = 0}^{n_{\overset{\sim}{y}}^{A}}\;{{N_{\theta,p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{N_{\xi,p_{\overset{\sim}{y}}}\left( \overset{\sim}{y} \right)}{\overset{\sim}{P}}_{\theta,\xi}^{A}}}}} \\{{= {\sum\limits_{\xi = 0}^{n_{\overset{\sim}{y}}^{A}}\;{{N_{\xi,p_{\overset{\sim}{y}}}\left( \overset{\sim}{y} \right)}\left( {\sum\limits_{\theta = 0}^{m_{\overset{\sim}{x}}}\;{{N_{\theta,p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{\overset{\sim}{P}}_{\theta,\xi}^{A}}} \right)}}},}\end{matrix}$where N_(θ,p) _({tilde over (x)}) denotes the θ-th NURBS basis functionof degree p_({tilde over (x)}), and N_(ξ,p) _({tilde over (x)}) denotesthe μ-th NURBS basis function of degree p_({tilde over (y)}). (In thespecial case where the curve portion CP_(MS,β) of the model space curveis a Bézier portion, the NURBS function N_(θ,p) _({tilde over (x)}) mayspecialize to the Bézier basis function B_(θ,p) _({tilde over (x)}) ofdegree p_({tilde over (x)}).) Note, in some embodiments, we mayinterpret the control points {tilde over (P)}_(θ,ξ) ^(A) as being pointsin 4D space, and recover 3D points by projection.)

Observe that the curve control points {{tilde over (Q)}_(ξ) ^(k):0≤ξ≤n_({tilde over (y)}) ^(A)} for this isocurve {tilde over (S)}_(iA,β)are given by:

${\overset{\sim}{Q}}_{\xi}^{k} = {\sum\limits_{\theta = 0}^{m_{\overset{\sim}{x}}}\;{{N_{\theta,p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{{\overset{\sim}{P}}_{\theta,\xi}^{A}.}}}$

For each ξ∈{1, 2, . . . , n_({tilde over (y)}) ^(A)} and each λ∈{1, 2, .. . , m_({tilde over (x)})−1}, we can set {tilde over (Q)}_(ξ) ^(k)equal to {tilde over (Q)}_(λ,ρ) ^(A), i.e., the ρ^(th) control point ofthe sub-isocurve RISO_({tilde over (x)}) _(λ) ^(A)({tilde over (y)}):

${\overset{\sim}{Q}}_{\lambda,\rho}^{A} = {\sum\limits_{\theta = 0}^{m_{\overset{\sim}{x}}}\;{{N_{\theta,p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{{\overset{\sim}{P}}_{\theta,\xi}^{A}.}}}$This strategy is reasonable since both the sub-isocurveRISO_({tilde over (x)}) _(λ) ^(A)({tilde over (y)}) and the isocurve{tilde over (S)}_(iA,β)(x_(k), y) are evaluated at {tilde over(x)}={tilde over (x)}_(λ).

In the above summation, we can separate out the terms (θ=0 andθ=m_({tilde over (x)})) containing known surface control points {tildeover (P)}_(0,ξ) ^(A) and {tilde over (P)}_(m) _({tilde over (x)},ξ) ^(A)from the terms (θ=1, 2, . . . , m_({tilde over (x)})−1) containingunknown surface control points, and obtain:

${{\overset{\sim}{Q}}_{\lambda,\rho}^{A} - {{N_{0,p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{\overset{\sim}{P}}_{0,\xi}^{A}} - {{N_{m_{\overset{\sim}{x}},p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{\overset{\sim}{P}}_{m_{\overset{\sim}{x}},\xi}^{A}}} = {\sum\limits_{\theta = 0}^{m_{\overset{\sim}{x}} - 1}\;{{N_{\theta,p_{\overset{\sim}{x}}}\left( {\overset{\sim}{x}}_{\lambda} \right)}{\overset{\sim}{P}}_{\theta,\xi}^{A}}}$Thus, for each ρ∈{1, 2, . . . , n_({tilde over (y)}) ^(A)−1} we have acorresponding determinate system of (m_({tilde over (x)})−1) linearvector equations in (m_({tilde over (x)})−1) vector unknowns, which maybe solved, e.g., using any standard technique from numerical linearalgebra, to determine the interior surface control points in the ξ-throw of the surface control net {{tilde over (P)}_(θ,ξ) ^(A)}, i.e., thesurface control points{{tilde over (P)} _(θ,ξ) ^(A):0≤θ≤m _({tilde over (x)})}.In other words, each interior row of the surface control net correspondsto a separate linear system, which may easily be solved to determine theinterior members of that row. Thus, the entire set of interior surfacecontrol points of the output patch {tilde over (S)}_(iA,β) are readilyobtained.

To our knowledge, this technique of solving for surface control pointsbased on isocurve control points is a new contribution to the theory oftensor product splines.

In some embodiments, the above described constraint-based pointrepositioning is applied to the control points of the sub-isocurves{RISO_({tilde over (x)}) _(λ) ^(A)({tilde over (y)})} before they areused to compute the surface control net of the output patch.

In some embodiments, we assume that at least the end control points

$\left\{ {{{\overset{\sim}{Q}}_{\lambda,n_{{\overset{\sim}{y}}^{i}}^{A}}^{A}\text{:}\mspace{11mu} 0} \leq \lambda \leq n_{{\overset{\sim}{y}}^{i}}^{A}} \right\}$of the sub-isocurves

$\left\{ {{R\; I\; S\;{O_{{\overset{\sim}{x}}_{\lambda}}^{A}\left( \overset{\sim}{y} \right)}\text{:}\mspace{11mu} 0} \leq \lambda \leq n_{{\overset{\sim}{y}}^{i}}^{A}} \right\}$have been repositioned so as to reside on the model space curve prior tocomputation of the surface control net of the output patch. In theseembodiments, the λ=n_({tilde over (y)}) _(i) ^(A) row of the surfacecontrol net may be addressed by linear system solution, just like thearbitrary interior row of the surface control net. Similarly, the λ=0row of the surface control net may be addressed by linear systemsolution, just like the arbitrary interior row of the surface controlnet. In some embodiments, all rows of the surface control net areaddressed by linear system solution.

In some embodiments, the Post-SSI algorithm may similarly construct thesurface control points for the output patch {tilde over (S)}_(iB,β)based on a set of isoparametric curve data including:

-   -   (a) the control points of the portion CP_(MS,β) of the model        space curve;    -   (b) the control points of the y^(i) sub-isocurves        {RISO_({tilde over (x)}) _(λ) ^(B)({tilde over (y)}        ^(i)):λ∈{0,1,2, . . . ,m _({tilde over (x)}) _(i) }},    -   or a subset of those control points.        In some embodiments, the set of isoparametric curve data may        also include the control points of the boundary isocurve ISO_(y)        _(LB) _(i) (x^(i)). In some embodiments, the set of        isoparametric curve data may include the interior control points        of the y^(i) sub-isocurves {RISO_({tilde over (x)}) _(λ)        ^(B)({tilde over (y)}^(i)): λ∈{0, 1, 2, . . . ,        m_({tilde over (x)}) _(i) }}, but exclude the end control points        of those sub-isocurves.

By employing the techniques disclosed above, the Post-SSI algorithm mayobtain a complete description of an output patch (e.g., the output patch{tilde over (S)}_(iA,β), or the output patch {tilde over (S)}_(iB,β), orthe combination of {tilde over (S)}_(iA,β) and S_(iB,β)) comprising asurface control net, a pair of knot vectors, a set of weights, and thedegrees of the output patch. This output patch may be interpreted as aportion (or the entirety of) a reconstructed output surface, e.g., aB-Spline surface, or a NURBS surface or a T-Spline surface.

Combining the Constructed Output Patch Together with LongitudinallySuperior and/or Inferior Patches of the Original Surface Geometry

Recall that the domain {tilde over (Ω)}_(β) ^(iA) of the output patch{tilde over (S)}_(iA,β) may be equal to the rectangle [{tilde over(x)}_(β) ^(i), {tilde over (x)}_(β+1) ^(i)]×[{tilde over (y)}_(min)^(iA), {tilde over (y)}_(max) ^(iA)]. In some embodiments, it ispossible that {tilde over (y)}_(min) ^(iA) is greater than y_(min) ^(i),i.e., the minimum value of parameter y^(i) in the domain Ω^(i) of thesurface S_(i). (To avoid possible confusion with {tilde over (y)}_(min)^(iA), we will here refer to y_(min) ^(i) as y_(floor) ^(i).) Thus, itmay be desirable to combine the output patch {tilde over (S)}_(iA,β)with the restriction of surface S_(i) to the longitudinally inferiordomainΩ_(β,Linf) ^(iA)=[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1)^(i)]×[y _(floor) ^(i) ,{tilde over (y)} _(min) ^(iA)],to obtain an extended output patch {tilde over (S)}_(iA,β) ^(ext) on thelongitudinally extended domain{tilde over (Ω)}_(β,LEXT) ^(iA)=[{tilde over (x)} _(β) ^(i) ,{tilde over(x)} _(β+1) ^(i)]×[y _(floor) ^(i) ,{tilde over (y)} _(max) ^(iA)],

This combination may be achieved by removing an appropriate number ofknots at {tilde over (y)}^(i)={tilde over (y)}_(min) ^(A). Thiscombination is easily achievable because the output surface patch {tildeover (S)}_(iA,β) has in at least some embodiments been designed torespect the geometry of surface S_(i) along its y^(i)=y_(min) ^(iA)boundary.

Furthermore, recall that the domain {tilde over (Ω)}_(β) ^(iB) of theoutput patch {tilde over (S)}_(iB,β) may be equal to the rectangle[{tilde over (x)}_(β) ^(i), {tilde over (x)}_(β+1) ^(i)]×[{tilde over(y)}_(min) ^(iB), {tilde over (y)}_(max) ^(iB)]. In some embodiments, itis possible that {tilde over (y)}_(max) ^(iB) is less than y_(max) ^(i),i.e., the maximum value of parameter y^(i) in the domain Ω^(i) of thesurface S_(i). (To avoid possible confusion with {tilde over (y)}_(max)^(iA), we will here refer to y_(max) ^(i) as y_(ceiling) ^(i).) Thus, itmay be desirable to combine the output patch {tilde over (S)}_(iB,β)with the restriction of surface S_(i) to the longitudinally superiordomainΩ_(β,Lsup) ^(iB)=[{tilde over (x)} _(β) ^(i) ,{tilde over (x)} _(β+1)^(i)]×[{tilde over (y)} _(max) ^(iB) ,y _(ceiling)].to obtain an extended output patch {tilde over (S)}_(iB,β) ^(ext) on thelongitudinally extended domain{tilde over (Ω)}_(β,LEXT) ^(iB)=[{tilde over (x)} _(β) ^(i) ,{tilde over(x)} _(β+1) ^(i)]×[{tilde over (y)} _(min) ^(iB) ,y _(ceiling) ^(i)].

This combination may be achieved by removing an appropriate number ofknots at {tilde over (y)}^(i)={tilde over (y)}_(max) ^(iB). Thiscombination is easily achievable because the output surface patch {tildeover (S)}_(iB,β) has been designed to agree with the geometry of surfaceS_(i) along its {tilde over (y)}^(i)={tilde over (y)}_(max) ^(iB)boundary.

5.8 Combining Output Patches for Successive Values of Portion Index β.

As noted above, a given segment (or cluster) of type x^(i) from theparameter space curve C_(PSi) may include a plurality of portionsCP_(MS,β) corresponding to respective values of the portion index β. Anoutput patch {tilde over (S)}_(iA,β) and/or an output patch {tilde over(S)}_(iB,β) may be constructed for each of the portions, as variouslydescribed above. It should be understood that successive output patches{tilde over (S)}_(iA,β) and {tilde over (S)}_(iA,β+1) (or, successiveoutput patches {tilde over (S)}_(iB,β) and {tilde over (S)}_(iB,β+1))associated with a given segment (or cluster) may be combined by applyingknot removal along their shared boundary. Thus, a single output patchmay be formed for each cluster (or segment).

Topology Reconstruction

The points, curves, and surfaces resulting from the above describedgeometry construction process are used to update the topology of theb-rep data structure. As described previously, this data structure willbe specific to the CAD package or exchange file type, yet the update isnot limited to any specific data structure and is amenable to any validb-rep data structure commonly used in CAD systems and CAD data exchangestandards.

In some embodiments, the boundary representation may be updated afterhaving constructed the output surface patch {tilde over (S)}_(iA,β)and/or the output surface patch {tilde over (S)}_(iB,β).

In some embodiments, the update may be performed as follows. The edge Emay be divided into two or more subedges {

_(j)}. One of those subedges, say

₀, points to the portion CP_(MS,β) of the model space curve, and theremaining subedge(s) point to remaining portion(s) of the model spacecurve. The original topological face F_(i) (associated with surfaceS_(i) and the original trim loop containing the parameter space trimcurve C_(PSi)) may be edited by adding an additional trim loop for the2D domain of surface S_(i), where the additional trim loop includesportion CP_(PSi,β) of the parameter space curve C_(PSi), and encloses aregion corresponding to the domain of the output surface patch {tildeover (S)}_(iA,β) (or the domain of the output surface patch {tilde over(S)}_(iB,β)). The orientation of the additional loop is selected toindicate that the said region no longer belongs to the face F_(i). A newtopological face {tilde over (F)}_(i) may be created and assigned theoutput surface patch {tilde over (S)}_(iA,β) (or the output surfacepatch {tilde over (S)}_(iB,β)) as its underlying surface spline object.The new topological face {tilde over (F)}_(i) may have four coedges:

-   _(i,0) ⁰, which corresponds to the {tilde over (x)}={tilde over    (x)}_(β) boundary of the output surface patch;-   _(i,1) ⁰, which corresponds to the {tilde over (x)}={tilde over    (x)}_(β+1) boundary of the output surface patch;-   _(i,2) ⁰, which corresponds to the {tilde over (y)}={tilde over    (y)}_(min) ^(iA) boundary of the output surface patch;-   _(i,3) ⁰, which corresponds to the {tilde over (y)}={tilde over    (y)}_(max) ^(iA) boundary of the output surface patch, i.e., the    boundary that meets the portion CP_(MS,β) of the model space curve.

The coedge

_(i,3) ⁰ may be identified with the subedge

₀. The remainder of the original surface patch will require an update toprovide appropriate topological adjacency information with the newsurfaces patches, {tilde over (S)}_(iA,β) and {tilde over (S)}_(iB,β),and their respective topological entities.

Output: B-rep CAD File

The new B-rep model is written out to the specified standard of the datastructure type of the CAD package and/or data format of the CAD file.

FIGS. 24A-B show two spline surfaces S₀ and S₁ that intersect in modelspace. The surface S₀ is a bicubic B-spline surface with internal knotsin both directions. The surface S₁ is a linearly lofted B-splinesurface, generated from a highly dynamic cubic curve with internalknots.

FIG. 25A illustrates an example of the result of a solid modelingBoolean operation with an SSI operation on spline surfaces S₀ and S₁shown in FIGS. 24A-B. FIG. 25B is a blowup of area 200 detailing thediscontinuous gap-replete fashion of the Boolean operation performed asshown in FIG. 25A.

FIGS. 26A-C illustrate the details of the result of the SSI operationshown in FIG. 25A in model space and the various parameter spaces.

FIG. 27A shows blowups of areas 300, 500, and 600 for spline surface S₀,with details. FIG. 27B shows blowups of areas 300, 700, and 800 forspline surface S₁, with details. FIG. 27C shows blowups of areas 300 and400, with details, along with a detail of the knot refined curve C_(MS).

FIG. 28 shows a detail of the knot refined curve C_(MS) with samplepoints shown. In this example the sample points are Greville points ofC_(MS).

FIGS. 29A-D shows isocurve sampling of surfaces S₀ and S₁, with bothversions of untrimmed and trimmed isocurves at C_(MS).

FIGS. 30A-D shows an example of linear isocurve reparameterization forisocurve sampling of surfaces S₀ and S₁.

FIG. 31A shows isocurve sampling of surface S₀ with its parameter space.FIG. 31B shows reconstructed T-spline surface {tilde over (S)}₀ with itsrespective parameter space. FIG. 31C shows reconstructed B-splinesurface {tilde over (S)}₀ with its respective parameter space. FIG. 31Dshows isocurve sampling of surface S₁ with its parameter space. FIG. 31Eshows reconstructed B-spline surface {tilde over (S)}₁ with itsrespective parameter space.

FIG. 32A shows T-spline surface {tilde over (S)} as a union of {tildeover (S)}₀ and {tilde over (S)}₁ with its global parameter space. FIG.32B shows B-spline surface {tilde over (S)} as a union of {tilde over(S)}₀ and {tilde over (S)}₁ with its global parameter space.

CAD Kernel Implementation

In one set of embodiments, we contemplate a dynamic method for operatingon spline surfaces to create corresponding output surfaces (ofcorresponding sets of output surfaces) so that: the boundary of eachoutput surface includes a model space curve that approximates thegeometric intersection of the spline surfaces; and each output surfacegeometrically approximates a trimmed version of the corresponding splinesurface. Thus, the output surfaces meet in a C⁰ continuous fashion alongthe model space curve. The dynamic method may operate in the kernel of aCAD software system, e.g., as part of a Boolean operation, and mayinclude many of the features described above in connection with thePost-SSI algorithm. However, the dynamic method may create the modelspace curve C_(MS) and the parameter space trim curves {C_(PSi)} insteadof receiving those curves as input data.

In some embodiments, the dynamic method may control the process ofcalculating the set of intersection points SOP_(MS), the sets of pointsSOP_(PS0) and SOP_(PS1). In particular, the dynamic method may arrangeto calculate these sets of points so as to furnish the requisite surfacereconstruction data in the most minimal and precise way possible.

Additional Procedures for the Construction of Spline Surfaces to ProvideInter-Surface Continuity

Global Parameter Space Reconstruction of Tensor Product Spline Surfacesby Nonlinear Reparameterization Surface Functions of IsoparametricCurves

Tensor product spline surfaces have come to dominate the geometricdatabases of modern Computer-Aided Design (CAD) applications. Used todescribe complicated real world models, intersecting spline surfaces areknown to create complex nonlinear exact solutions of a very high degreewhen Boolean operations are performed. Referred to as thesurface-surface intersection (SSI) issue, or the trim problem, CADapplications have addressed this matter by approximating theseintersections, thus introducing gaps between the discrete surfaces.Methods to reconstruct spline surfaces at trim edges resulting inwatertight models seek to represent the users' design intent, as well asmaking the model suitable for downstream processing in computationalanalysis and manufacturing. This process should reconstruct therepresentation in both geometric and parametric terms, although oftenonly the geometric portion is considered. Alternatively, herein wedescribe an admissible global parameter space created from the originaltensor product splines in order to build such watertightrepresentations. The continuity afforded from this achieves bothparametric and geometric considerations when coupled with a propergeometric surface reconstruction algorithm.

Tensor product spline surface reconstruction is a process aimed atrebuilding gap-ridden boundary representation (b-rep) solid models suchthat they result in watertight representations at the trim andintersection edges. Because of the surface-surface intersection (SSI)algorithms used in approximating the interface between the surfacejunctions, a massive effort is required to make this connectionconformal for downstream processing in fields such as computationalanalysis, computer-aided manufacturing (CAM), animation, metrology orcomputer-aided inspection (CAI), etc. Steps to achieve automated surfacehealing, as well as Boolean operations that are initially watertight,may require a means of mapping parameter spaces from the originallydisconnected domains to that of a globally coupled form.

The mappings between distinct parameterizations sampled from b-reps mayemploy well documented reparameterization schemes on individualisoparametric curves to achieve the goal of global parameterreconstruction. If not handled correctly, these mappings have thepotential of introducing artifacts between adjacent reconstructedsegments. Herein we describe a novel approach that seeks the nonlinearreparameterization functions for entire sets of isoparametric curves.These reparameterization functions are themselves isoparametric curves,sampled from a nonlinear global reparameterization surface function. Inthis way, given the requisite input for the system, optimal mappings forthe system of isoparametric curves can be performed by manipulating thedegrees of freedom of the reparameterization surface. The descriptionand use of this reparameterization surface defined below is a new andunpublished concept in the field of Computer-Aided Geometric Design(CAGD).

The processes below relate to transforming parameter spaces from a groupof independent surface definitions from a b-rep solid model to a singlewatertight definition. We take the isoparametric curve element as thecenterpiece of this operation in order to relate this conversion ofinformation in a complete and reliable fashion. Using well-knownreparameterization operations established for spline curves, we describehow these operations are extended for the purpose of reparametrizingsets of isoparametric curves. This is shown to not simply be anextension of this procedure as a batch process, but a concept andtechnique replete with its own unique constraints and degrees offreedom.

Solving the systems of linear equations that arise from this frameworkis a computationally interesting challenge itself and below wedemonstrate one of many possible means by utilizing a very commoninterpolation strategy shown to be appropriate for the task. While otherinterpolation strategies may work just as well, we focus on cubic curvesgiven their ubiquity in CAD systems, although this does not limit theability to handle curves of other degrees, nor the ability to implementother interpolation strategies for any degree. The method may be fullyintegrated within a CAD application making execution streamlined.

Isoparametric Curves, Surface Extraction, and ReparameterizationTechniques

The concept of an isoparametric curve, or isocurve, is a ubiquitouselement in Computer-Aided Geometric Design (CAGD) utilized frequently inthe art of surface and solid modeling [Piegl 1997]. Isoparametric curvescan be derived from any standard tensor product surface definition, suchas Bézier, B-spline, NURBS or T-spline surfaces, and are abundantlydocumented in CAGD references. Without loss of generality, assume anarbitrary B-spline surface S=S(s, t) defined on a domain Ω=[s₀,s_(m+p+1)]×[t₀, t_(n+q+1)]⊂

², with knot vectors

and

, of degrees p in s and q in t, respectively,

${S\left( {s,t} \right)} = {\sum\limits_{\lambda = 0}^{m}\;{\sum\limits_{\rho = 0}^{n}\;{{N_{\lambda,p}(s)}{N_{\rho,q}(t)}{P_{\lambda,\rho}.}}}}$

Here, P_(λ,ρ) are the surface control points specified at arbitrarilyuser-defined locations. An isocurve is a spline based curve extractedfrom a surface along one of its parametric values. It is created bysetting the other parametric value of the surface to a constant, i.e.,s=s, at the value where the intended isocurve is to be created,

${{ISO}_{\overset{\_}{s}}(t)} = {{S\left( {\overset{\_}{s},t} \right)} = {\sum\limits_{\lambda = 0}^{m}\;{\sum\limits_{\rho = 0}^{n}\;{{N_{\lambda,p}\left( \overset{\_}{s} \right)}{N_{\rho,q}(t)}{P_{\lambda,\rho}.}}}}}$

Because s=s, a known input value, the basis functions N_(λ,p)(s) can befully evaluated, with no mathematical approximation (as opposed to theinevitable issues of numerical precision in computing). Grouping all ofthe known values of the expression together we find

${{ISO}_{\overset{\_}{s}}(t)} = {\sum\limits_{\rho = 0}^{n}\;{{N_{\rho,q}(t)}{\left( {\sum\limits_{\lambda = 0}^{m}\;{{N_{\lambda,p}\left( \overset{\_}{s} \right)}P_{\lambda,\rho}}} \right).}}}$

Let us define the sum of evaluated basis functions with control pointsto be,

${Q_{\rho} = {{Q_{\rho}\left( \overset{\_}{s} \right)} = {\sum\limits_{\lambda = 0}^{m}\;{{N_{\lambda,p}\left( \overset{\_}{s} \right)}P_{\lambda,\rho}}}}},$where the values Q_(ρ) represent point values. Thus,

${{{ISO}_{\overset{\_}{s}}(t)} = {\sum\limits_{\rho = 0}^{n}\;{{N_{\rho,q}(t)}Q_{\rho}}}},$which is comparable to the standard B-spline curve equation,

${C(u)} = {\sum\limits_{\zeta = 0}^{n}\;{{N_{\zeta,p}(u)}{P_{\zeta}.}}}$

The above equation is that of an isoparametric spline curve (isocurve)embedded within the surface that traces out an image along a constantsurface parameter value of the surface from which it is extracted. Thisis not an approximation; rather, this is mathematically an exactrepresentation by reformulation. Note that the curve control points,Q_(ρ), are distinct from the surface control points, P_(λ,ρ). Also notethat the isocurves above were evaluated in the t spanning direction, yetsimilar equations may be derived for the s spanning direction bychoosing t=t. FIG. 33 shows all the isocurves evaluated for the internalknots of an arbitrarily defined surface S₀(s⁰, t⁰), for the valid b-repsolid model of intersecting surfaces S₀(s⁰, t⁰) and S₁(s¹, t¹).

For the purposes of surface reconstruction in making watertight models,we seek to employ a method using isocurves on surfaces at the points ofthe model space trim curve C_(MS)(v_(β)) that correspond to theparameter values v_(β) of the curve. This trim curve has been Bézierrefined such that v_(β) represents the projections of all internalsurface knots, critical points of the parameter space trim curves fromeach surface, and all other knots indicative of trim feature data. Theseisocurves correspond to the surface parameter boundary values s_(β) ^(i)and s_(β+1) ^(i) of each curve portion of these refined segmentsCP_(MS,β)(v_(β)) and, along with user defined lower and upper boundparameters, t_(LB) ^(i) and t_(UB) ^(i), respectively, define anextraction domain Ω_(β) ^(i) for each surface, as shown with the examplein FIG. 34.

Furthermore, isocurves are sought on each surface at the respectiveGreville abscissae of the refined segments, CP_(MS,β)(v_(γ)), indicatedby s_(γ) ^(i). Each extraction domain Ω_(β) ^(i) will include a supersetof isocurve parameter values at the curve boundaries and Grevilleabscissae, with the most common case of an interpolating cubic trimcurve yielding surface parameters s_(λ) ^(i)={s_(β) ₀ ^(i), s_(γ) ₀^(i), s_(γ) ₁ ^(i), s_(β) ₁ ^(i)}.

For the extraction domain Ω_(β) ^(i) shown in the example depicted inFIG. 34, the isocurves at these points are shown in FIG. 35, split atthe trim curve. As detailed in this diagram, having sampled theisocurves, we modify the isocurves to reflect the trim boundary. Eachisocurve ISO_(s) _(λ) _(i) (t^(i)) is knot refined at the locationt^(i)=t_(SSI,λ) ^(i) as determined based on the model space curve (orthe portion CP_(MS,β) of the model space curve). Specifically, t_(SSI,λ)^(i) is computed as the value of the v_(λ) parameter that achieves theclosest approach of the isocurve ISO_(s) _(λ) _(i) (t^(i)) to the modelspace curve C_(MS)(v). In other words, the distance between ISO_(s) _(λ)_(i) (t^(i)) and C_(MS)(v) is minimized over the space {(v, t^(i))}.ISO_(s) _(λ) _(i) (t^(i)), refined to the knot multiplicity of theisocurve at t^(i)=t_(SSI,λ) ^(i), divides the isocurve into twoisocurves, denoted ISO_(s) _(λ) _(i) ^(A)(t^(i)) and ISO_(s) _(λ) _(i)^(B)(t^(i)).

Having defined an extraction domain and then sampled and dividedisocurves from the extraction domain, the isocurves need to bereparametrized such that they are defined on a single knot vector domainthat defines the watertight reconstruction surface. Having extracted acollection of isocurves from the above procedures, it is typically thecase that different isocurves of the set {ISO_(s) _(λ) _(i) ^(A)(t^(i)):λ=0, 1, 2, . . . , m_(s) _(i) } have different numbers of knots, in bothvalue and multiplicity. The most obvious source of differences in theknots of such isocurve sets is that the isocurves are typically ofdifferent lengths, and thus, a longer isocurve may have a longer span aswell as intercept a larger number of transverse knot lines of thesurface patch S_(i,β) than a shorter isocurve. Such is the case in FIG.35, as we note that not only do the knots of the isocurves vary inmultiplicity, but also the shared knot values exist at the samelocations for different knot span intervals. The result is that, whennormalized, none of these knot values will agree within the collectionof isocurves when each of their domains is uniquely scaled. This is avery important detail with regards to the continuity of thereconstructed surface patches. Forming a shared surface domain from thisassemblage of non-matching knot vectors is an extremely difficult,non-trivial task. We resolve the situation by using the common procedureof reparameterization with a unique and innovative technique.

We seek to reparametrize the isocurves ISO_(s) _(λ) _(i) ^(A)(t^(i)) onthe surface patch S_(i,β) ^(A). The reparameterization operation isconcerned with mapping the parameter space of a curve to a new domainwith its own knot vector while maintaining the exact geometric trace inmodel space. The parametric conversion requires a reparameterizationfunction whose construction and mapping will concern us herein. Thereparameterization function may be linear, piecewise linear, ornon-linear, and may be computed in a multitude of ways, which willdistinctively contribute attributes of the corresponding isocurve afterbeing reparametrized.

We state the reparameterization operation precisely by seeking afunction t^(i)=ƒ_(s) _(λ) _(i) ^(A)({tilde over (t)}^(i)) that maps fromthe closed interval I_(β)=[t_(β), t_(β+1)] onto the intervalĨ_(β)=[{tilde over (t)}_(β),{tilde over (t)}_(β+1)] such that, for all tin I_(β), there exists a {tilde over (t)} in Ĩ_(β) that produces thesame set of points in

³ when evaluated as a model space curve, ISO_(s) _(λ) _(i) ^(A)({tildeover (t)}^(i)). Another way of stating this is that the Hausdorffdistance is zero between the sets of points ISO_(s) _(λ) _(i)^(A)({tilde over (t)}^(i)) and the set of points ISO_({tilde over (s)})_(λ) _(i) ^(A)({tilde over (t)}^(i)). In order for thereparameterization function to ensure that the new parameter intervalĨ_(β) is mapped in a one-to-one fashion from I_(β), t^(i)=ƒ_(s) _(λ)_(i) ^(A)({tilde over (t)}^(i)) must be strictly monotonic.

The most direct mathematical formulation of this condition is acomposite function ISO_(s) _(λ) _(i) ^(A)(t^(i))=ISO_({tilde over (s)})_(λ) _(i) ^(A)(ƒ_(s) _(λ) _(i) ^(A)({tilde over (t)}_(i))). Theresulting isocurve of this composition would then be a complicated,arbitrary function of an unpredictable form that could not be guaranteedas representable in a conventional CAD system. As an alternative, weseek reparameterization functions that are themselves B-splines, andform a procedure to create a B-spline representation of the resultingisocurve, ISO_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i)).

Isocurve Linear Reparameterization

The first method of defining the reparameterization function t^(i)=ƒ_(s)_(λ) _(i) ^(A)({tilde over (t)}^(i)) is computationally the easiest,linear reparameterization. While trivial to compute, it results in alarge number of knot insertions when applied to B-spline reconstruction.FIG. 36 illustrates the discussion that follows.

We noted above that if we simply normalized each isocurve ISO_(s) _(λ)_(i) ^(A)(y^(i)), λ∈{0, 1, 2, . . . , m_(x) _(i) } to a commonparametric interval I_({tilde over (s)}) _(i) ^(iA)=[{tilde over(t)}_(min) ^(iA), {tilde over (t)}_(max) ^(iA)] in the t^(i) parameter,we could potentially have a collection of isocurves whose knot vectorscontain wildly divergent sets of knots. Let us go ahead and perform thisoperation to obtain a corresponding linearly reparametrized set ofisocurves LRISO_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i)).(Here, “LR” distinguishes the reparameterization method as linear.) Notethat there are a wide variety of ways to select {tilde over (t)}_(min)^(iA) and {tilde over (t)}_(max) ^(iA). For example, we may chooseI_({tilde over (s)}) _(i) ^(iA)=[0,1]. In another version,I_({tilde over (s)}) _(i) ^(iA)=[t_(LB) ^(i), {tilde over (t)}_(SSI)^(i)], where {tilde over (t)}_(SSI) ^(i) is strictly between t_(LB) ^(i)and t_(UB) ^(i), e.g., the midpoint of t_(LB) ^(i) and t_(UB) ^(i).

Having linearly reparametrized the isocurves for each λ∈{0, 1, 2, . . ., m_(s) _(i) }, we will then identify any knots of the reparametrizedisocurve LRISO_({tilde over (s)}) _(μ) _(i) ^(A)({tilde over (y)}_(i)),μ∈{0, 1, 2, . . . , m_(s) _(i) }, μ≠λ, that are not present in the knotvector of the reparametrized isocurve LRISO_(s) _(λ) _(i) ^(A)({tildeover (t)}^(i)), and add those identified knots to the knot vector ofLRISO_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i)). In otherwords, wherever one reparametrized isocurve has a knot that is absentfrom the knot vector of a different reparametrized isocurve, we add thatknot to the knot vector of the different reparametrized isocurve. As aresult, each of the reparametrized isocurves will share a common knotvector and hence may be interpreted as an isocurve of the reconstructedB-spline surface.

Isocurve Nonlinear Reparameterization

In a nonlinear reparameterization scheme of the t^(i) isocurves, eachisocurve ISO_(s) _(λ) _(i) ^(A)(t^(i)), λ∈{0, 1, 2, . . . , m_(s) _(i)}, is reparametrized with a corresponding reparameterization functiont^(i)=ƒ_(s) _(k) _(i) ^(A)({tilde over (t)}^(i)), to obtain acorresponding reparametrized isocurve NLRISO_({tilde over (s)}) _(λ)_(i) ^(A)({tilde over (t)}^(i)), so that the reparametrized isocurves{NLRISO_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i))} aredefined on a common parametric interval I_({tilde over (s)}) _(i)^(iA)=[{tilde over (t)}_(min) ^(iA), {tilde over (t)}_(max) ^(iA)] andhave a common knot vector

^(i). The functions {ƒ_(s) _(λ) _(i) ^(A)} are 1D splines of degreep_(REPARAM) (nonlinear reparameterization implies p_(REPARAM) is greaterthan one), in which case the isocurves {NLRISO_({tilde over (s)}) _(λ)_(i) ^(A)({tilde over (t)}^(i))} will be of degree where p_(t) _(i) isthe degree of the isocurves {ISO_(s) _(λ) _(i) ^(A)(t^(i))}.

One method of simplifying this process is to use the longest of theisocurves as the model for the final form of the reparametrized knotvector with the goal of obtaining isocurves defined on a common knotvector. FIG. 37 details an example of this procedure. Let s_(LONG) ^(iA)denote the value of s_(λ) ^(i) corresponding to the longest isocurve.Each remaining isocurve ISO_(s) _(λ) _(i) ^(A)(t^(i)), s_(λ)^(i)≠s_(LONG) ^(iA), can then be nonlinearly reparametrized with acorresponding reparameterization function t^(i)=ƒ_({tilde over (s)})_(λ) _(i) ^(A)({tilde over (t)}^(i)) of common degree p_(REPARAM), toobtain a corresponding nonlinearly reparametrized isocurve, denotedEVRIS0_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i)) of degreep_(t) _(i) p_(Reparam) (the prefix “EV” is meant as a reminder of theelevation of the longest isocurve). The longest isocurve can then bedegree elevated to degree p_(t) _(i) p_(Reparam), to obtain adegree-elevated isocurve, denoted

${EVRIS}\; 0_{{\overset{\sim}{s}}_{LONG}^{iA}}^{A}{\left( {\overset{\sim}{t}}^{i} \right).}$The nonlinear reparameterizations and the degree elevation are performedin such a fashion that the resulting set of m_({tilde over (s)}) _(i) +1isocurves {EVRIS0_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over(t)}^(i)): λ∈{0, 1, 2, . . . , m_({tilde over (s)}) _(i) }} are ofdegree p_(t) _(i) p_(Reparam) and share a common knot vector. Forexample, if p_(t) _(i) =3, p_(Reparam)=3, and the isocurve

ISO_(s_(m_(s^(i)))^(i))^(A)(t^(i))is the longest of the isocurves, then each of the isocurves ISO_(s) _(λ)_(i) ^(A)(t^(i)), λ∈{1, 2, . . . , m_(s) _(i) } can be nonlinearlyreparametrized with a corresponding degree 3 reparameterization splinefunction

${t^{i} = {f_{{\overset{\sim}{s}}_{\lambda}^{i}}^{A}\left( {\overset{\sim}{t}}^{i} \right)}},$and the isocurve

ISO_(s_(m_(s^(i)))^(i))^(A)(t^(i))can be degree elevated to degree 9, to obtain a common knot vector. Herewe do not have to deal with the exchange of differing knots betweenisocurves, as in the case of linear reparameterization, as the presentnon-linear mapping is constructed specifically to avoid this. Thedownside is that the curves are now of a higher degree, and in the caseof the example go from degree 3 to degree 9.Global Reparameterization Functions

We will now form the domain {tilde over (Ω)}_(β) ^(iA) for the outputpatch {tilde over (S)}_(iA,β). This domain may be interpreted as areconstruction domain because the output patch {tilde over (S)}_(iA,β)is constructed so as to rebuild (or modify) the geometry based oninformation extracted from the surface patch S_(i,β) on the extractionsubdomain Ω_(β) ^(iA). The reconstruction subdomain {tilde over (Ω)}_(β)^(iA) becomes a subregion of the global parameter domain {tilde over(Ω)}^(i). The reconstruction subdomain {tilde over (Ω)}_(β) ^(iA) isdefined to be [{tilde over (s)}_(β), {tilde over (s)}_(β+1)^(i)]×[{tilde over (t)}_(min) ^(iA), {tilde over (t)}_(max) ^(iA)],where {tilde over (t)}_(min) ^(iA) and {tilde over (t)}_(max) ^(iA) arerespectively the first and last knot values of the common knot vector ofthe reparametrized isocurves.

Note that similar processing operations could be applied to theisocurves on the other side of the model space curve, i.e., on theisocurves {ISO_(s) _(λ) _(i) ^(B)(t^(i)): λ=0, 1, 2, . . . , m_(s) _(i)}. There are no fundamental differences based on which side of the modelspace trim curve the extraction domain is taken from, and so everythingstated in this chapter applies for all k∈{A, B}.

In the following discussion of reparameterization, it is not significanthow the isocurves {ISO_(s) _(λ) _(i) ^(A)(t^(i))} have beenreparametrized to a common knot vector. Thus, we may drop the prefix“L”, “NL” and “EV”, and refer simply to reparametrized isocurves{RISO_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i))} todistinguish them from the original configuration {ISO_(s) _(λ) _(i)^(A)(t^(i))}. This notation is meant to encompass any type ofreparameterization being applied to the isocurves to obtainreparametrized isocurves that share a common knot vector.

Piecewise Domain Reparameterization

The global parameter domain {tilde over (Ω)}^(iA) could be formed bycollecting the individually formed subdomains {tilde over (Ω)}_(β)^(iA). The collection of reconstructed subdomains {tilde over (Ω)}_(β)^(iA) are difficult to create independently while maintaining anamenable parametric boundary. This is to say that forming subdomains{tilde over (Ω)}_(β) ^(iA) and {tilde over (Ω)}_(β+1) ^(iA) separatelydoes not guarantee a valid space {tilde over (Ω)}^(iA) when completedmeeting either the definition of a B-spline or the desired continuity ofglobal reparameterization construction. Each of the independentsubdomains would have to be checked and modified at their boundaries,parametric and geometric, in order to ensure a conformal edge betweendiscrete subdomains as well as continuity requirements between theindividual patches. This is a tedious and unnecessary process as a morecomprehensive process exists that is more appropriate.

Global Parameter Space Reparameterization and Reconstruction

The piecewise reparameterization and assembly operations above will nowbe combined into a novel synthesis. FIG. 38 depicts the global parameterspace reconstruction and reparameterization as per the same example usedin the previous figures. Instead of these operations taking placeindependently for each isocurve s_(λ) ^(i), introducing the possibilityof error and incompatibility if not monitored and regulated, we canconstrain the task to not only guarantee valid output, but also tofacilitate the users' understanding of the non-unique solution space andthe input information for which they are required to supply.

We previously stated that in a nonlinear reparameterization scheme ofthe t^(i) isocurves, each isocurve ISO_(s) _(λ) _(i) ^(A)(t^(i)), λ∈{0,1, 2, . . . , m_(x) _(i) }, is reparametrized with a correspondingreparameterization function t^(i)=ƒ_(s) _(k) _(i) ^(A)({tilde over(t)}^(i)), to obtain a corresponding reparametrized isocurveNLRISO_({tilde over (s)}) _(λ) _(i) ^(A)({tilde over (t)}^(i)), so thatthe reparametrized isocurves {NLRISO_({tilde over (s)}) _(λ) _(i)^(A)({tilde over (t)}^(i))} are defined on a common parametric intervalI_({tilde over (s)}) _(i) ^(iA)=[{tilde over (t)}_(min) ^(iA), {tildeover (t)}_(max) ^(iA)] and have a common knot vector

^(i). We state the trivial fact that the linear reparameterizationscheme is the simplest of these functions so as to generalize all thereparameterization schemes by use of a reparameterization function,t^(i)=ƒ_({tilde over (x)}) _(i) ({tilde over (t)}^(i)). As noted before,without loss of generality, that we prefer this function to be a 1-Dspline function that maps one univariate parameter domain correspondingto an isocurve to another. With respect to the reparameterizationfunction, we make a self-referential observation here, mirroring that ofthe observation of the isocurve extraction. If the unified isocurvecollection originates from a single surface with a shared domain, and wewish to form a congruent collection of 1-D spline reparameterizationfunctions, we can define a parent spline reparameterization surface fromwhich these isocurves originate. In a symmetric operation (i.e. inreverse) to the isocurve extraction process, the definition of thereparameterization surface serves as the users' guide to thereparameterization process, constrained appropriately within thevalidity and continuity restrictions of the mapping.

Constraints and Degrees of Freedom

The reparameterization surface will be defined as a bivariatereparameterization function t^(i)=ƒ^(k)({tilde over (s)}^(i), {tildeover (t)}^(i)), where ƒ^(k) takes on the form of a tensor product splinesurface itself for all k∈{A, B}. Here, the knot vectors of t^(i) and{tilde over (t)}^(i) must be defined to reflect the original surfacedomain Ω^(i) as well as the prescribed domain {tilde over (Ω)}^(i),respectively. Each subdomain, Ω_(β) ^(i) and {tilde over (Ω)}_(β) ^(i),is then satisfied locally, with the interface boundaries coincident ascomponents of the continuous surface construction. Control pointlocations of the reparameterization surface may be user specified.Furthermore, the reparameterization function ƒ^(k) can take a linearform or any of an infinite number of nonlinear forms, under thefollowing conditions:

-   -   1. t^(i)=ƒ^(k)({tilde over (s)}^(i), {tilde over (t)}^(i)) must        be strictly monotonic in the {tilde over (t)}^(i) direction so        as to map the parameter interval without overlap;    -   2. t^(i)=ƒ^(k)({tilde over (s)}^(i), {tilde over (t)}^(i)) as a        spline must be built using the same target knot vector,        ^(i), of the reconstructed parameter space in the {tilde over        (t)}^(i) direction;    -   3. Each distinct knot value t_(σ) ^(i)∈        ^(i) must be mapped to a distinct knot value in {tilde over        (t)}_(k) ^(i)∈        ^(i) such that the reparameterization function t_(σ)        ^(i)=ƒ^(k)({tilde over (s)}_(λ) ^(i), {tilde over (t)}_(k) ^(i))        interpolates the point made by these knot values at each value        of {tilde over (s)}_(λ) ^(i).

FIG. 38 depicts an example showing the relationship between thereparameterization surface t^(i)=ƒ^(k) ({tilde over (s)}^(i), {tildeover (t)}^(i)) and the individual 1-D reparameterization functionst^(i)=ƒ_({tilde over (s)}) _(λ) _(i) ^(k)({tilde over (t)}^(i)) for eachisocurve of a subdomain {tilde over (Ω)}_(β) ^(i). To understand thereparameterization surface as a tensor product spline, its degrees offreedom, and the isocurve reparameterization functions extracted, westate this as follows,

${{S_{r}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = \begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\{f^{k}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)}\end{bmatrix}}},{{S_{r}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} = {\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}P_{\delta,\mu}}}}},{{S_{r}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = \begin{bmatrix}{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}\gamma_{\delta,\overset{\sim}{s}}^{i}}}} \\{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}\gamma_{\mu,\overset{\sim}{t}}^{i}}}} \\{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}t_{\delta,\mu}}}}\end{bmatrix}}},$where γ_(δ,{tilde over (s)}) ^(i) and γ_(μ,{tilde over (t)}) ^(i) arethe Greville abscissae in the global reparameterization space andt_(δ,μ) are specified from the sampled isocurve domain, the originalparameter space. This reduces to,

${S_{r}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = {\begin{bmatrix}{{\overset{\sim}{s}}_{\lambda}^{i}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} \\{{\overset{\sim}{t}}_{\lambda}^{i}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} \\{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}t_{\delta,\mu}}}}\end{bmatrix}.}}$

Note several details of these equations. First, that {tilde over(s)}_(λ) ^(i)({tilde over (s)}^(i), {tilde over (t)}^(i)) and {tildeover (t)}_(λ) ^(i)({tilde over (s)}^(i), {tilde over (t)}^(i)) are fixedfunctions as defined by the Greville points γ_(δ,{tilde over (s)}) ^(i)and γ_(μ,{tilde over (t)}) ^(i), respectively, and that no furtherinformation must be provided to evaluate them. Next, we find thatspecification of the reparameterization surface becomes thespecification of spline attributes, i.e., degree, knot vector, etc., anda set of scalar values, t_(δ,μ), that specify the height field of,

${f^{k}\left( {{\overset{\sim}{s}}^{i},{\overset{\sim}{t}}^{i}} \right)} = {\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}{t_{\delta,\mu}.}}}}$

For a specified value of {tilde over (s)}_(λ) ^(i) corresponding to theλ-th isocurve, the reparameterization surface function becomes areparameterization isocurve function and the univariatereparameterization for the curve is defined as,

${{C_{r}\left( {\overset{\sim}{t}}^{i} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = \begin{bmatrix}{\overset{\sim}{s}}_{\lambda}^{i} \\{\overset{\sim}{t}}^{i} \\{f_{s^{i}}^{k}\left( {\overset{\sim}{t}}^{i} \right)}\end{bmatrix}}},{{C_{r}\left( {\overset{\sim}{t}}^{i} \right)} = {\sum\limits_{\zeta = 0}^{n}\;{{N_{\zeta,p}\left( {\overset{\sim}{t}}^{i} \right)}P_{\zeta}}}},{{C_{r}\left( {\overset{\sim}{t}}^{i} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = \begin{bmatrix}{\overset{\sim}{s}}_{\lambda}^{i} \\{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}_{\lambda}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}\gamma_{\mu,\overset{\sim}{t}}^{i}}}} \\{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}_{\lambda}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}t_{\delta,\mu}}}}\end{bmatrix}}},{{C_{r}\left( {\overset{\sim}{t}}^{i} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = {\begin{bmatrix}{\overset{\sim}{s}}_{\lambda}^{i} \\{{\overset{\sim}{t}}_{\lambda}^{i}\left( {{\overset{\sim}{s}}_{\lambda}^{i},{\overset{\sim}{t}}^{i}} \right)} \\{\sum\limits_{\delta = 0}^{m}\;{\sum\limits_{\mu = 0}^{n}\;{{N_{\delta,p}\left( {\overset{\sim}{s}}_{\lambda}^{i} \right)}{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}t_{\delta,\mu}}}}\end{bmatrix}.}}}$

Just like the definition of an isocurve from a surface, because {tildeover (s)}_(λ) ^(i) is known and N_(δ,p)({tilde over (s)}_(λ) ^(i)) canbe evaluated, the resulting coefficients could be combined with thecontrol points,

${C_{r}\left( {\overset{\sim}{t}}^{i} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}^{i} \\{\overset{\sim}{t}}^{i} \\t^{i}\end{bmatrix} = {\begin{bmatrix}{\overset{\sim}{s}}_{\lambda}^{i} \\{\sum\limits_{\mu = 0}^{n}\;{{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}\gamma_{{\overset{\sim}{s}}_{\lambda}^{i},\mu}^{i}}} \\{\sum\limits_{\mu = 0}^{n}\;{{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}t_{{\overset{\sim}{s}}_{\lambda}^{i},\mu}}}\end{bmatrix}.}}$

Again, {tilde over (t)}_(λ) ^(i)({tilde over (s)}_(λ) ^(i), {tilde over(t)}^(i)) is fixed from the Greville points γ_({tilde over (s)}) _(λ,μ)_(i) ^(i) and that no further information must be provided to evaluateit. Next, we find that specification of the reparameterization curvebecomes the specification of spline attributes and a set of scalarvalues, t_({tilde over (s)}) _(λ,μ) _(i) , that specify the height fieldof,

${f_{s^{i}}^{k}\left( {\overset{\sim}{t}}^{i} \right)} = {\sum\limits_{\mu = 0}^{n}\;{{N_{\mu,q}\left( {\overset{\sim}{t}}^{i} \right)}{t_{{\overset{\sim}{s}}_{\lambda}^{i},\mu}.}}}$

This description and its utilization as such a resource has not beenemployed to date and represents an exciting new tool. Besides theprescription of the original domain data and the strict monotonicity ofthe surface function, the control point locations and the constructionof the new domain are up to the user.

Cubic Spline Interpolation Approach

In the above equations, given that we constrain particular knots of theoriginal domain to map to knot values of the target reconstructiondomain, i.e., t_(ρ) is mapped to {tilde over (t)}_(ρ), we are theninterested in finding an interpolating spline solution through points,

${S\left( {{\overset{\sim}{s}}_{\xi}^{i},{\overset{\sim}{t}}^{i}} \right)} = {\begin{bmatrix}{\overset{\sim}{s}}_{\xi}^{i} \\{\overset{\sim}{t}}_{\rho}^{i} \\t_{\rho}\end{bmatrix}.}$

To find the unknown values of t_(δμ) that will then define thereparameterization functions, we note that there already existsinterpolation procedures for the given conditions described above. Forcubic spline curves, one common interpolation strategy can be easilyaccomplished for curves where the knot values are coincident with theparameters used to evaluate the interpolation points. Because thereparameterization function must be built from the same knot vector

^(i) in {tilde over (t)}^(i) we seek the following solution:

$\begin{bmatrix}{{S\left( {{\overset{\sim}{s}}_{\xi}^{i},{\overset{\sim}{t}}_{1}^{i}} \right)} - {a_{1,\xi}P_{1,\xi}}} \\{S\left( {{\overset{\sim}{s}}_{\xi}^{i},{\overset{\sim}{t}}_{2}^{i}} \right)} \\\vdots \\{S\left( {{\overset{\sim}{s}}_{\xi}^{i},{\overset{\sim}{t}}_{n - 2}^{i}} \right)} \\{{S\left( {{\overset{\sim}{s}}_{\xi}^{i},{\overset{\sim}{t}}_{n - 1}^{i}} \right)} - {c_{{n - 1},\xi}P_{{n + 1},\xi}}}\end{bmatrix} = {\quad{{\begin{bmatrix}b_{1,\xi} & c_{1,\xi} & 0 & \ldots & 0 & 0 & 0 \\a_{2,\xi} & b_{2,\xi} & c_{2,\xi} & \ldots & 0 & 0 & 0 \\\vdots & \vdots & \vdots & \ddots & \vdots & \cdots & \vdots \\0 & 0 & 0 & \ldots & a_{{n - 2},\xi} & b_{{n - 2},\xi} & c_{{n - 2},\xi} \\0 & 0 & 0 & \ldots & 0 & a_{{n - 1},\xi} & b_{{n - 1},\xi}\end{bmatrix}\begin{bmatrix}P_{2,\xi} \\P_{3,\xi} \\\vdots \\P_{{n - 1},\xi} \\P_{n,\xi}\end{bmatrix}},\mspace{20mu}{where},{a_{k,\xi} = {N_{k,3}\left( {\overset{\sim}{t}}_{k}^{i} \right)}},{b_{k,\xi} = {N_{{k + 1},3}\left( {\overset{\sim}{t}}_{k}^{i} \right)}},{c_{k,\xi} = {{N_{{k + 2},3}\left( {\overset{\sim}{t}}_{k}^{i} \right)}.}}}}$

Since all of the values of {tilde over (t)}_(k) ^(i) are knowninterpolation values, the values of the surface control points P_(k,ξ)can be solved with an (n−1)×(n−1) linear system for each ξ isoparametriccurve.

The above solution strategy may require certain user inputs:

-   -   1. Input geometry to sample relevant data from;    -   2. The target knot vector definition;    -   3. Height values for interpolation from the target knot vector        locations where an explicit value from the original knot vector        is not specified.

These values can be made dynamic for user manipulation in real-time orset to be automatically optimized for particular target criteria. Thecubic curve interpolation strategy adopted here for creating thereparameterization surface is reliable, and is not the only methodavailable.

By switching from a B-spline form to a T-spline form, the choices for aknot vector become numerous, with another infinite set of admissibleconfigurations. Certainly defining metrics to optimize would focus thissearch to a more reasonable and bounded process.

As there is one (n−1)×(n−1) linear system for each isocurve to be solvedin creating the reparameterization surface, given the uncoupled natureof the individual systems of equations, the solution could beparallelized in any number of ways, in many different areas of thehardware. Having this speed is essential for users interested inrealtime manipulation of the system variables. As shown, the benefit tothis is that tight integration into existing CAD applications ispossible such that many existing tools offered in such packages can beutilized.

Pre-SSI Algorithmic Detail

The following paragraphs give additional algorithmic detail specific tothe Pre-SSI WatertightCAD methodology.

In one set of embodiments, an algorithm may operate on a pair ofsurfaces that have not undergone a conventional Boolean operation (i.e.not an existing boundary representation solid model of an object repletewith gaps at surface-surface intersections as a result of a conventionalsolid modeling Boolean operation). This algorithm serves to provide therequisite information required for processes related to global parameterspace formation and surface reconstruction to provide inter-surfacecontinuity. Hereinafter, this algorithm may be referred to as thePre-SSI WatertightCAD algorithm.

The input for the Pre-SSI WatertightCAD algorithm may be spline surfaceswith the intention of performing a solid modeling Boolean operation.(The spline surfaces may include tensor product spline surfaces such asNURBS surfaces and/or T-Spline surfaces.) In one embodiment, the splinesurfaces may be provided in a CAD file. In another embodiment, thePre-SSI WatertightCAD algorithm may operate as part of the kernel of aCAD system, in which case the spline surfaces may have a specializedinternal format.

There are many different ways to realize spline surfaces in terms of adata structure. (Different CAD packages have different data structurerealizations.) Furthermore, there are many different file formats usedto represent the spline surface data. For execution of the Pre-SSIWatertightCAD algorithm, the data structure type and/or data file formatof the CAD file need not be of any specific standard, whether a productbased type or interoperability standard. Some examples of data fileformats include, but are not limited to, ACIS (*.sat, *.sab), STEP(*.stp, *.step), Rhino (*.3dm, *.3dmbak), etc.

The Pre-SSI WatertightCAD algorithm may operate on parametricallydefined spline surfaces of an object, and not on alternativerepresentations such as polytope meshes (structured or unstructured),constructive solid geometry (CSG) data, and implicit geometricrepresentations.

The Pre-SSI WatertightCAD algorithm may operate on the surface splinesS₀ and S₁ so as to produce the minimum data required for later processesthat create a first set of one or more output surface patches and asecond set of one or more output surface patches, where a boundary of aunion of images of the first set of output surface patches and aboundary of a union of the images of the second set of output surfacepatches meet in a C⁰ continuous fashion along the model space curveC_(MS). Thus, the gaps in a conventional Boolean surface-surfaceintersection operation will neither have been created nor need to berepaired. Given the original surface data, output surface patches areconstructed to meet in a C⁰ continuous fashion at the design intentBoolean intersection.

The spline surfaces {S_(i)} may be tensor product spline surfaces ofdegree combination p_(i)×q_(j), where p_(i) and q_(i) are integersgreater than zero, where p_(i) denotes the degree in one parametricdimension, and q_(i) denotes the degree in the other parametricdimension. For example, in some embodiments p_(i)=q_(i)=3, i.e., thesurfaces are bicubic surfaces. In other embodiments, the surfaces may bebiquartic or biquintic surfaces, or have mixed degrees. In someembodiments, the surfaces may have different degree combinations. Forexample, one surface might be bicubic while another surface isbiquartic. The spline surface S_(i)=S_(i)(s^(i), t^(i)) is defined on adomainΩ^(i)=[s _(min) ^(i) ,s _(max) ^(i)]×[t _(min) ^(i) ,t _(max) ^(i)]⊂

²,and has knot vectors

^(i) and

^(i).

$\begin{matrix}{S_{0} = {{S_{0}\left( {s^{0},t^{0}} \right)} = \begin{bmatrix}{x^{0}\left( {s^{0},t^{0}} \right)} \\{y^{0}\left( {s^{0},t^{0}} \right)} \\{z^{0}\left( {s^{0},t^{0}} \right)} \\{w^{0}\left( {s^{0},t^{0}} \right)}\end{bmatrix}}} \\{= {\sum\limits_{i = 0}^{m_{0}}\;{\sum\limits_{j = 0}^{n_{0}}\;{{N_{i,p_{0}}\left( s^{0} \right)}{N_{j,q_{0}}\left( t^{0} \right)}P_{ij}^{0}}}}}\end{matrix}$ ${where},{P_{ij}^{0} = {\begin{bmatrix}x_{ij}^{0} & y_{ij}^{0} & z_{ij}^{0} & w_{ij}^{0}\end{bmatrix}^{T}.\begin{matrix}{S_{1} = {{S_{1}\left( {s^{1},t^{1}} \right)} = \begin{bmatrix}{x^{1}\left( {s^{1},t^{1}} \right)} \\{y^{1}\left( {s^{1},t^{1}} \right)} \\{z^{1}\left( {s^{1},t^{1}} \right)} \\{w^{1}\left( {s^{1},t^{1}} \right)}\end{bmatrix}}} \\{= {\sum\limits_{i = 0}^{m_{1}}\;{\sum\limits_{j = 0}^{n_{1}}\;{{N_{i,p_{1}}\left( s^{1} \right)}{N_{j,q_{1}}\left( t^{1} \right)}P_{ij}^{1}}}}}\end{matrix}}}$ ${where},{P_{ij}^{1} = \begin{bmatrix}x_{ij}^{1} & y_{ij}^{1} & z_{ij}^{1} & w_{ij}^{1}\end{bmatrix}^{T}}$Algorithm Description

The Pre-SSI WatertightCAD algorithm includes a sequence of operationsthat take spline surfaces as input and output the necessary data forglobal reparameterization and surface reconstruction, specifically,surface isocurves at specific locations and a knot vector V defining thesurface-surface intersection.

Each surface S_(i) may furnish specific points solved for in theirrespective parameter spaces, commonly referred to as significant pointsor characteristic points. These significant points (s^(i), t^(i))_(α)include border points (where the parameter space curve meets the domainboundaries), singular points and turning points (at which curve tangentsare parallel to the parametric axes, also referred to as stationary andnon-stationary critical points). The significant points (s^(i),t^(i))_(α) will produce model space points S_(i)(s^(i), t^(i))_(α) atthe surface-surface intersection through evaluation of the surfacemapping S_(i).

Significant points may be solved by first implicitizing one of thebivariate parametric spline surfaces in the surface-surface intersectionoperation,

${H_{0}\left( {x,y,z,w} \right)} = {{\sum\limits_{i = 0}^{2p_{0}q_{0}}\;{\sum\limits_{j = 0}^{{2p_{0}q_{0}} - i}\;{\sum\limits_{k = 0}^{{2p_{0}q_{0}} - i - j}\;{D_{ijk}^{0}x^{i}y^{j}z^{k}w^{\deg - i - j - k}}}}} = 0}$${or},{{H_{1}\left( {x,y,z,w} \right)} = {{\sum\limits_{i = 0}^{2p_{1}q_{1}}\;{\sum\limits_{j = 0}^{{2p_{1}q_{1}} - i}\;{\sum\limits_{k = 0}^{{2p_{1}q_{1}} - i - j}\;{D_{ijk}^{1}x^{i}y^{j}z^{k}w^{\deg - i - j - k}}}}} = 0}}$

Having made implicit one of the bivariate parametric spline surfaces, weseek the surface-surface intersection equation between a surface inparametric form and the other surface in implicit algebraic form: (S₀ ∩H₁) or (S₁ ∩ H₀). As the parametric equation is a vector of equations inthe model space coordinates (x, y, z), the surface-surface intersectionmerely becomes a substitution of these equations into the model spacecoordinate variables in the implicit algebraic equation,H ₀(x ¹(s ¹ ,t ¹),y ¹(s ¹ ,t ¹),z ¹(s ¹ ,t ¹),w ¹(s ¹ ,t ¹))=0or,H ₁(x ⁰(s ⁰ ,t ⁰),y ⁰(s ⁰ ,t ⁰),z ⁰(s ⁰ ,t ⁰),w ⁰(s ⁰ ,t ⁰))=0

Significant points are determined using the above surface-surfaceintersection equation between a surface in parametric form and the otherin implicit algebraic form. This is achieved through differentiation andthen the solving of a simple system of equations:

${{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 2}},{\Omega^{0} = \left\{ {{\begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{\frac{\partial H_{1}}{\partial s^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial s^{0}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 2}},{\Omega^{1} = \left\{ {{\begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{\frac{\partial H_{0}}{\partial s^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial s^{1}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 3}},{\Omega^{0} = \left\{ {{\begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{\frac{\partial H_{1}}{\partial t^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial t^{0}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 3}},{\Omega^{1} = \left\{ {{\begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{\frac{\partial H_{0}}{\partial t^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial t^{1}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 0}},{\Omega^{0} = \left\{ {{\begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{\frac{\partial H_{1}}{\partial s^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial s^{0}} = 0}} \\{\frac{\partial H_{1}}{\partial t^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial t^{0}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 0}},{\Omega^{1} = \left\{ \begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{\frac{\partial H_{0}}{\partial s^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial s^{1}} = 0}} \\{\frac{\partial H_{0}}{\partial t^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial t^{1}} = 0}}\end{matrix} \right.}} \right.}} \right.}} \right.}} \right.}} \right.}$

Next, significant points of a new type are determined by similar methodsusing the above surface-surface intersection equation between a surfacein parametric form and the other in implicit algebraic form. This isalso achieved through differentiation and then the solving of a simplesystem of equations, yet has never been documented within literature ofthe field. These significant points are labeled as type 1, and referredto as inflection points of H_(i), where an inflection point is a pointon the locus H_(i)=0 such that the slope of H_(i) relative to surfaceparameter s^(j)(j≠i) is equal in absolute value to the slope of H_(i)relative to surface parameter t^(j):

${\frac{\partial{H_{i}\left( {s^{j},t^{j}} \right)}}{\partial s^{j}}} = {\frac{\partial{H_{i}\left( {s^{j},t^{j}} \right)}}{\partial t^{j}}}$This definition of an intersection point on H_(i) that is also asignificant point of type 1 results in the following equations andsubtypes,

$\begin{matrix}{{{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.0}},{\Omega^{0} = \left\{ \begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{\frac{\partial H_{1}}{\partial s^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial s^{0}} = a}} \\{\frac{\partial H_{1}}{\partial t^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial t^{0}} = a}}\end{matrix} \right.}} \\{= \left\{ \begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{{\frac{\partial H_{1}}{\partial s^{0}} - \frac{\partial H_{1}}{\partial t^{0}}} = 0}\end{matrix} \right.}\end{matrix}$ $\begin{matrix}{{{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.0}},{\Omega^{1} = \left\{ \begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{\frac{\partial H_{0}}{\partial s^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial s^{1}} = a}} \\{\frac{\partial H_{0}}{\partial t^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial t^{1}} = a}}\end{matrix} \right.}} \\{= \left\{ \begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{{\frac{\partial H_{0}}{\partial s^{1}} - \frac{\partial H_{0}}{\partial t^{1}}} = 0}\end{matrix} \right.}\end{matrix}$ $\begin{matrix}{{{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.1}},{\Omega^{0} = \left\{ \begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{\frac{\partial H_{1}}{\partial s^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial s^{0}} = a}} \\{\frac{\partial H_{1}}{\partial t^{0}} = {\frac{\partial{H_{1}\left( {s^{0},t^{0}} \right)}}{\partial t^{0}} = {- a}}}\end{matrix} \right.}} \\{= \left\{ \begin{matrix}{H_{1} = {{H_{1}\left( {s^{0},t^{0}} \right)} = 0}} \\{{\frac{\partial H_{1}}{\partial s^{0}} + \frac{\partial H_{1}}{\partial t^{0}}} = 0}\end{matrix} \right.}\end{matrix}$ $\begin{matrix}{{{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.1}},{\Omega^{1} = \left\{ \begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{\frac{\partial H_{0}}{\partial s^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial s^{1}} = a}} \\{\frac{\partial H_{0}}{\partial t^{1}} = {\frac{\partial{H_{0}\left( {s^{1},t^{1}} \right)}}{\partial t^{1}} = {- a}}}\end{matrix} \right.}} \\{= \left\{ \begin{matrix}{H_{0} = {{H_{0}\left( {s^{1},t^{1}} \right)} = 0}} \\{{\frac{\partial H_{0}}{\partial s^{1}} + \frac{\partial H_{0}}{\partial t^{1}}} = 0}\end{matrix} \right.}\end{matrix}$

Alternatively, significant points of type 1 may be determined by makinga simple change of parameters and then solving the resulting system inthe same as that as significant points 2 and 3 above. The change ofparameters is a simple 2-D rigid rotation in parameter space of theamount

${\frac{\pi}{4}\mspace{11mu}{radians}} = {45\mspace{14mu}\text{degrees:}}$

${\overset{\overset{\_}{\_}}{s}}^{0} = {{{s^{0}{\cos\left( \frac{\pi}{4} \right)}} - {t^{0}{\sin\left( \frac{\pi}{4} \right)}}} = {\left( \frac{\sqrt{2}}{2} \right)\left( {s^{0} - t^{0}} \right)}}$${\overset{\overset{\_}{\_}}{t}}^{0} = {{{s^{0}{\sin\left( \frac{\pi}{4} \right)}} + {t^{0}{\cos\left( \frac{\pi}{4} \right)}}} = {\left( \frac{\sqrt{2}}{2} \right)\left( {s^{0} + t^{0}} \right)}}$and therefore,

$s^{0} = {{{{\overset{\overset{\_}{\_}}{s}}^{0}{\cos\left( \frac{\pi}{4} \right)}} + {{\overset{\overset{\_}{\_}}{t}}^{0}{\sin\left( \frac{\pi}{4} \right)}}} = {\left( \frac{\sqrt{2}}{2} \right)\left( {{\overset{\overset{\_}{\_}}{s}}^{0} + {\overset{\overset{\_}{\_}}{t}}^{0}} \right)}}$$t^{0} = {{{{- {\overset{\overset{\_}{\_}}{s}}^{0}}{\sin\left( \frac{\pi}{4} \right)}} + {{\overset{\overset{\_}{\_}}{t}}^{0}{\cos\left( \frac{\pi}{4} \right)}}} = {\left( \frac{\sqrt{2}}{2} \right)\left( {{- {\overset{\overset{\_}{\_}}{s}}^{0}} + {\overset{\overset{\_}{\_}}{t}}^{0}} \right)}}$

The implicit algebraic functions become,

${{\overset{\overset{\_}{\_}}{H}}_{0}\left( {{\overset{\overset{\_}{\_}}{S}}_{1}\left( {{\overset{\overset{\_}{\_}}{s}}^{1},{\overset{\overset{\_}{\_}}{t}}^{1}} \right)} \right)} = 0$${{\overset{\overset{\_}{\_}}{H}}_{1}\left( {{\overset{\overset{\_}{\_}}{S}}_{0}\left( {{\overset{\overset{\_}{\_}}{s}}^{0},{\overset{\overset{\_}{\_}}{t}}^{0}} \right)} \right)} = 0$${{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.0}},{\Omega^{0} = \left\{ {{\begin{matrix}{{\overset{\overset{\_}{\_}}{H}}_{1} = {{{\overset{\overset{\_}{\_}}{H}}_{1}\left( {{\overset{\overset{\_}{\_}}{s}}^{0},{\overset{\overset{\_}{\_}}{t}}^{0}} \right)} = 0}} \\{\frac{\partial{\overset{\overset{\_}{\_}}{H}}_{1}}{\partial{\overset{\overset{\_}{\_}}{s}}^{0}} = {\frac{\partial{{\overset{\overset{\_}{\_}}{H}}_{1}\left( {{\overset{\overset{\_}{\_}}{s}}^{0},{\overset{\overset{\_}{\_}}{t}}^{0}} \right)}}{\partial{\overset{\overset{\_}{\_}}{s}}^{0}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.0}},{\Omega^{1} = \left\{ {{\begin{matrix}{{\overset{\overset{\_}{\_}}{H}}_{0} = {{{\overset{\overset{\_}{\_}}{H}}_{0}\left( {{\overset{\overset{\_}{\_}}{s}}^{1},{\overset{\overset{\_}{\_}}{t}}^{1}} \right)} = 0}} \\{\frac{\partial{\overset{\overset{\_}{\_}}{H}}_{0}}{\partial{\overset{\overset{\_}{\_}}{s}}^{1}} = {\frac{\partial{{\overset{\overset{\_}{\_}}{H}}_{0}\left( {{\overset{\overset{\_}{\_}}{s}}^{1},{\overset{\overset{\_}{\_}}{t}}^{1}} \right)}}{\partial{\overset{\overset{\_}{\_}}{s}}^{1}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.1}},{\Omega^{0} = \left\{ {{\begin{matrix}{{\overset{\overset{\_}{\_}}{H}}_{1} = {{{\overset{\overset{\_}{\_}}{H}}_{1}\left( {{\overset{\overset{\_}{\_}}{s}}^{0},{\overset{\overset{\_}{\_}}{t}}^{0}} \right)} = 0}} \\{\frac{\partial{\overset{\overset{\_}{\_}}{H}}_{1}}{\partial{\overset{\overset{\_}{\_}}{t}}^{0}} = {\frac{\partial{{\overset{\overset{\_}{\_}}{H}}_{1}\left( {{\overset{\overset{\_}{\_}}{s}}^{0},{\overset{\overset{\_}{\_}}{t}}^{0}} \right)}}{\partial{\overset{\overset{\_}{\_}}{t}}^{0}} = 0}}\end{matrix}{Significant}\mspace{14mu}{points}\text{:}\mspace{14mu}{type}\mspace{14mu}{\alpha 1}{.1}},{\Omega^{1} = \left\{ \begin{matrix}{{\overset{\overset{\_}{\_}}{H}}_{0} = {{{\overset{\overset{\_}{\_}}{H}}_{0}\left( {{\overset{\overset{\_}{\_}}{s}}^{1},{\overset{\overset{\_}{\_}}{t}}^{1}} \right)} = 0}} \\{\frac{\partial{\overset{\overset{\_}{\_}}{H}}_{0}}{\partial{\overset{\overset{\_}{\_}}{t}}^{1}} = {\frac{\partial{{\overset{\overset{\_}{\_}}{H}}_{0}\left( {{\overset{\overset{\_}{\_}}{s}}^{1},{\overset{\overset{\_}{\_}}{t}}^{1}} \right)}}{\partial{\overset{\overset{\_}{\_}}{t}}^{1}} = 0}}\end{matrix} \right.}} \right.}} \right.}} \right.}$

The choice of equations for determining significant points of type 1depends on numerical techniques involved in root determination andcomputational methods for solving systems of equations. The above showsthat multiple options exist, with variants, which are both simple andconvenient within current CAD software implementations, the optimalbeing operational, system, and application specific.

The significant points above can be sequenced in a topologically correctordered set giving the correct series of significant points along thesurface-surface intersection. Techniques for ordering a given set ofpoints along a surface-surface intersection are common within both CAGDliterature as well as commercial applications. Based on existing pointordering methods, we form the following paired sets:

Parameter space Ω⁰:Ψ⁰={(s ⁰ ,t ⁰)_(0α) ⁰, . . . ,(s ⁰ ,t ⁰)_(jα) ⁰, . . . ,(s ⁰ ,t ⁰)_((N)₀ _(−1)α) ⁰};j=0, . . . ,(N ₀−1)

Model space S₀:Γ⁰ ={S ₀(s ⁰ ,t ⁰)_(0α) ⁰ , . . . ,S ₀(s ⁰ ,t ⁰)_(jα) ⁰ , . . . ,S ₀(s ⁰,t ⁰)_((N) ₀ _(−1)α) ⁰};j=0, . . . ,(N ₀−1)

Parameter space Ω¹:Ψ¹={(s ¹ ,t ¹)_(0α) ¹, . . . ,(s ¹ ,t ¹)_(jα) ¹, . . . ,(s ¹ ,t ¹)_((N)₁ _(−1)α) ¹};j=0, . . . ,(N ₁−1)

Model space S₁:Γ¹ ={S ₁(s ¹ ,t ¹)_(0α) ¹ , . . . ,S ₁(s ¹ ,t ¹)_(jα) ¹ , . . . ,S ₁(s ¹,t ¹)_((N) ₁ _(−1)α) ¹};j=0, . . . ,(N ₁−1)Here the superscript on each point indicates the domain in which thesignificant point was solved in, while the subscript a indicates thesignificant point type from those previously described above.

Classification of sections of Ψ^(i) and Γ^(i).

Each successive significant point in sets Ψ^(i) and Γ^(i) corresponds toa pair with its previous respective value in the set, this orderingpartitioning the sets into segments based on each pair of points.

Each segment, whether in Ψ^(i) or Γ^(i), may be classified as being oftype s^(i) or type t^(i) based on the types of its endpoints, e.g., asshown in FIG. 39. If the segment is bounded by significant points(s^(i), t^(i))_(jα) ^(i) and (s^(i), t^(i))_((j+1))α^(i), there aresixteen possible cases for the ordered pair(Type(s ^(i) ,t ^(i))_(jα) ^(i),Type(s ^(i) ,t ^(i))_((j+1))α^(i)),where Type(s^(i), t^(i))_(jα) ^(i) denotes the type of the significantpoint (s^(i), t^(i))_(jα) ^(i), and Type(s^(i), t^(i))_((j+1)α) ^(i)denotes the type of the significant point (s^(i), t^(i))_((j+1)α) ^(i).As shown in FIG. 39, the type of the segment is immediately resolved incertain cases of those sixteen cases, which are labeled with s^(i) ort^(i). In other cases, the type of the segment must be checked asfollows. Calculate the velocity vector

$\frac{{dt}^{i}}{{ds}^{i}} = \left\lbrack {\frac{\partial H_{k}}{\partial s^{i}},\frac{\partial H_{k}}{\partial t^{i}}} \right\rbrack^{T}$for k≠i, at any point in the interior of the segment (i.e., excludingthe end points), and classify the segment based on:

${{Segment}\mspace{14mu}{Type}} = \left\{ {\begin{matrix}{{{type}\mspace{14mu} s^{i}},{{{if}\mspace{11mu}{{{dt}^{i}/{ds}^{i}}}} \leq 1}} \\{{{type}\mspace{14mu} t^{i}},{{{if}\mspace{11mu}{{{dt}^{i}/{ds}^{i}}}} > 1}}\end{matrix}.} \right.$Note that the choice of assigning the boundary case |dt^(i)/ds^(i)|=1 totype s^(i) is arbitrary. It could equally well have been assigned totype t^(i).

Furthermore, observe that the segment type classification is invariantunder exchange of the endpoint types. For example, the segment type ofpair (2,1) is the same as the segment type of pair (1,2). Finally, notethat the type pairs (3,2) and (2,3) are not possible if the segmentationhas been performed properly. (Each represents a transition between zeroslope and infinite slope, which will not occur on a continuous curvesegment without an intermediate significant point of type 1.) Thus, anerror warning may be issued if either of these type pairs isencountered.

Clustering Segments of Same Type into Intervals

The following paragraphs provide additional detail regarding step 212from FIG. 13, wherein segments are clustered in the Pre-SSI algorithm.

In some embodiments, each maximal contiguous group of segments of thesame type may be combined to form a cluster (i.e., interval) of the sametype. This creates ordered subsets within each of the ordered sets Ψ^(i)and Γ^(i).

As an example, given a sequence of six segments having the correspondingsequence of types {s^(i), s^(i), s^(i), t^(i), t^(i), s^(i)}, then thefirst three segments may be combined to form a cluster of type s^(i);the next two segments may be combined to form a cluster of type t^(i);and the last segment would constitute a singleton cluster of type s^(i).Assuming, arbitrarily, that this sequence of segments is on the domainΩ⁰, this results in the following sets of points:

Ψ⁰ = {{(s⁰, t⁰)_(0 α)⁰, (s⁰, t⁰)_(1α)⁰, (s⁰, t⁰)_(2α)⁰, (s⁰, t⁰)_(3α)⁰}₀^(s⁰), {(s⁰, t⁰)_(3α)⁰, (s⁰, t⁰)_(4α)⁰, (s⁰, t⁰)_(5α)⁰}₁^(t⁰), {(s⁰, t⁰)_(5α)⁰, (s⁰, t⁰)_(6α)⁰}₂^(s⁰)}Γ⁰ = {{S₀(s⁰, t⁰)_(0 α)⁰, S₀(s⁰, t⁰)_(1α)⁰, S₀(s⁰, t⁰)_(2α)⁰, S₀(s⁰, t⁰)_(3α)⁰}₀^(s⁰), {S₀(s⁰, t⁰)_(3α)⁰, S₀(s⁰, t⁰)_(4α)⁰, S₀(s⁰, t⁰)_(5α)⁰}₁^(t⁰), {S₀(s⁰, t⁰)_(5α)⁰, S₀(s⁰, t⁰)_(6α)⁰}₂^(s⁰)}Note here that the subset superscript denotes the classification whilethe subscript denotes the cluster.

In some embodiments, certain aspects of the processing to be describedbelow may be performed on the basis of a cluster.

Check for Extraordinary Point Locations at Singular Points

As part of the classification and clustering operation, theWatertightCAD algorithm may identify a point of degeneracy as follows.Let vector vel_(a) denote the velocity vector of the surface-surfaceintersection curve in parameter space H_(k)(s^(i), t^(i)), k≠i, at anidentified point (s^(i)±δ, t^(i)±δ)_(jα0→(j−1)α) ^(i), near a singularpoint (s^(i), t^(i))_(jα0) ^(i), in between (s^(i), t^(i))_(jα0) ^(i)and (s^(i), t^(i))_((j−1)α) ^(i). Let vector vel_(b) denote the velocityvector of the surface-surface intersection curve in parameter spaceH_(k)(s^(i), t^(i)), k≠i, at an identified point (s^(i)±δ,t^(i)±δ)_(jα0→(j+1)α′) ^(i), near a singular point (s^(i), t^(i))_(jα0)^(i), in between (s^(i), t^(i))_(jα0) ^(i) and (s^(i), t^(i))_((j+1)α′)^(i).

${{vel}_{a} = {\frac{{dt}^{i}}{{ds}^{i}}\left( {{s^{i} \pm \delta},{t^{i} \pm \delta}} \right)_{{j\;{\alpha 0}}\rightarrow{{({j - 1})}\alpha}}^{i}}},{{vel}_{b} = {\frac{{dt}^{i}}{{ds}^{i}}{\left( {{s^{i} \pm \delta},{t^{i} \pm \delta}} \right)_{{j\;{\alpha 0}}\rightarrow{{({j + 1})}\alpha}}^{i}.}}}$

Then, the point (s^(i), t^(i))_(jα0) ^(i) may be identified as anextraordinary point if vel_(a) and vel_(b) satisfy the following angularcondition:

${{135{^\circ}} \leq {\cos^{- 1}\left( \frac{{vel}_{a} \cdot {vel}_{b}}{{{vel}_{a}}{{vel}_{b}}} \right)} \leq {225{^\circ}}},$where vel_(a)·vel_(b) denotes the dot product of vectors vel_(a) andvel_(b), and ∥vel_(a)∥ and ∥vel_(b)∥ denote the Euclidean norm ofvectors vel_(a) and vel_(b), respectively.

Add points to subsets of Ψ^(i) and Γ^(i) to reflect the internal knotss^(i) or t^(i) of S_(i), depending on classification subsets.

For a given subset cluster of Ψ^(i) or Γ^(i), the spline surface S_(i)may contribute additional points to these sets from interior surfaceknots of the spline surface S_(i), depending on the classification ofthe segment. Suppose x^(i)∈{s^(i), t^(i)} denotes the type of thecluster for a subset of Ψ^(i) or Γ^(i). Then all x_(min_α)^(i)≤x^(i)≤x_(max_α) ^(i) admit new points to the subset as (x^(i),y^(i))^(i), where [x_(min_α) ^(i), x_(max_α) ^(i)] bound the values ofthe subset in the classified variable. (An x^(i) knot of the surfaceS_(i) is either an s^(i) knot if the segment is classified as a clusterof type s^(i), or a t^(i) knot if the segment is classified as a clusterof type t^(i).)

As an example, given a subset of the following points,Ψ_(subset_j) ^(i)={(1.1,0.5)_(0α) ^(i),(2.7,3.1)_(1α)^(i),(3.4,0.9)_(2α) ^(i),(6.2,2.7)_(3α) ^(i)}_(j) ^(s) ^(i) ,for the knot vectors,

^(i)=[−2.4,−0.2,0.5,1.2,3.5,5.6,7.6,8.4,9.3]and

^(i)=[−5.3,−2.1,−1.1,0.6,1.9,2.2,6.3,10.2,12.5]as the subset is classified as s^(i), additional knot values from

^(i) of 1.1≤1.2, 3.5, 5.6≤6.2 need to be added to Ψ_(subset_j) ^(i)after their respective t^(i) values have been solved for at thesurface-surface intersection. This is accomplished by solving thedeterminate system of surface-surface intersection equation wheres^(i)=s ^(i) has been specified. This may also be interpreted as theintersection of the isocurve of surface at S_(i) (s ^(i), t^(i)) withS_(k)(s^(k), t^(k)), k≠i, which is a directly solvable mathematicalproblem, commonly articulated and offered in current CAD applications.This not only furnishes t^(i)=t ^(i) but also s^(k)=s ^(k) and t^(k)=t^(k). In the example above, the updated subset becomes,Ψ_(subset_j) ^(i)={(1.1,0.5)_(0α) ^(i),(1.2, t ^(i))_(s) _(i)^(i),(2.7,3.1)_(1α) ^(i),(3.4,0.9)_(2α) ^(i),(3.5, t ^(i))_(s) _(i)^(i),(5.6, t ^(i))_(s) _(i) ^(i),(6.2,2.7)_(3α) ^(i)}_(j) ^(s) ^(i)with respective model space point set,Γ_(subset_j) ^(i) ={S _(i)(1.1,0.5)_(0α) ^(i) ,S _(i)(1.2, t ^(i))_(s)_(i) ^(i) ,S _(i)(2.7,3.1)_(1α) ^(i) ,S _(i)(3.4,0.9)_(2α) ^(i) ,S_(i)(3.5, t ^(i))_(s) _(i) ^(i) ,S _(i)(5.6, t ^(i))_(s) _(i) ^(i) ,S_(i)(6.2,2.7)_(3α) ^(i)}_(j) ^(s) ^(i)

From the above methodology, points are to be added to all subsets ofΨ^(i) and Γ^(i) to reflect the internal knots s^(i) or t^(i) of S_(i),depending on bounds and classification of

^(i) and T^(i), as specified based on classification subsets index.

Merging sets Γ⁰ and Γ¹ to Create Model Space Point Set with which toPerform Curve Interpolation.

We now seek to merge Γ⁰ and Γ¹ into a properly ordered set with the endgoal of having a point set with which to perform curve interpolation,approximating the surface-surface intersection curve in model space.

For each point S_(i) (s^(i), t^(i))_(jα or x) _(i) ^(i) in Γ^(i), weseek the “pull-back” or “projection” into the complementary parameterspace, Ω^(k), k≠i. Procedures for doing so are numerical, as therequisite inverse functions are not solved for. These are common inprocedures in CAGD literature as well as current CAD applications. Wewill represent the operation by its notational operation,{dot over (Ψ)}^(k) ={S _(k) ⁻¹(S _(i)(s ^(i) ,t ^(i))_(jα or x) _(i)^(i)),k≠i}.

Having solved for {dot over (Ψ)}⁰ and {dot over (Ψ)}¹ we now seek thefollowing sets,{umlaut over (Ψ)}^(k)={dot over (Ψ)}^(k)∪Ψ^(k)where {umlaut over (Ψ)}^(k) is a properly ordered set reflecting themerged order of {dot over (Ψ)}^(k) and Ψ^(k). If the sets have beenmerged correctly, the topology indicated by the ordered points of{umlaut over (Ψ)}⁰ and {umlaut over (Ψ)}¹ should be identical. Thisorder shall be the order of the set of model space points {umlaut over(Γ)}^(k)=Γ⁰ ∪ Γ¹.

The ordered set {umlaut over (Γ)}^(k) is used to create a univariateparametric curve C_(MS)(v) interpolating the sequence of points in thisset as an approximation of the model space surface-surface intersectioncurve, in the process generating a knot vector V that contains uniqueknots describing points {umlaut over (Γ)}^(k) on C_(MS)(v). We refer tothese knots v_(β) as originating from the significant point locations onthe surfaces v_(α) as well as the internal surface knots v_(x) _(i) .There are plenty of methods to accomplish this interpolation andparametric curve formation, as well as refining and subdividing thiscurve to achieve an approximation of the surface-surface intersection toa user specified tolerance. Any of these methods can be employed tofurnish additional knots that are specific to the description of theparametric curve v_(ψ).

Having generated a knot vector V, this will be used to form the globalreparametrized space for the reconstructed surfaces. This will also beused to sample the necessary isocurves required for these processes. TheGreville abscissae v_(γ) of V serve as locations on C_(MS)(v) to findpoints to sample the requisite isocurves. Points on the model spacecurve C_(MS)(v_(γ)) may be mapped to a surface knot x _(γ) ^(i) ontoS_(i)(x _(γ) ^(i), y_(γ) ^(i)) via a minimization of the distancebetween the point and surface. This is accomplished by solving thedeterminate system of surface-surface intersection equations wherex^(i)=x _(γ) ^(i) has been specified. This may also be interpreted asthe intersection of the isocurve of surface at S_(i)(x _(γ) ^(i), y_(γ)^(i)) with S_(k)(x^(k), y^(k)), k≠i, which is a directly solvablemathematical problem, commonly articulated and offered in current CADapplications. These points will be referred to as P_(γ) and used tofurnish isocurves at the necessary locations.

Having created knot vector V and having solved for the surface-surfaceintersection points P_(γ), the purpose of the Pre-SSI WatertightCADalgorithm has been fulfilled for downstream processes. In someembodiments, one novel aspect of the Pre-SSI WatertightCAD algorithm isthe exactness and compactness of generating the minimum amount ofinformation required from original objects. This is due to solving forthe isocurve sampling points directly, using theoretically accurate CAGDequations with common computational methods found within CAD systems.This data does not depend on intermediary elements from other operations(i.e., conventional b-rep solid modeling Boolean operations), therebymaking the WatertightCAD algorithm a process in which data is derivedfrom the input data with the least amount of processing required. Suchmethods have been absent from both literature in the field as well astechniques in current applications.

Parameterization Change Handling Using Embedded Extensions, InsertedExtraordinary Points, and C⁰ Points

The following paragraphs present additional algorithmic detail regardingthe parameterization change handling, step 310 of FIG. 15, of theWatertightCAD methodology.

In the processing of both Pre-SSI and Post-SSI algorithms, a necessarystep occurs such that changes in parameterization are handled to matchthe intent of the user regarding the final form of the parameterizationand control point organization of the reconstructed geometry. Multipleschemes are available, distinguished by the resulting continuity,complexity, and structure. Hereinafter, this algorithm may be referredto as the parameter change handling algorithm.

Both Pre-SSI and Post-SSI algorithms result in the same input to theparameter change handling algorithm. This input data includes a knotvector V with knots of distinct types forming specifically clusteredsegments, and surface-surface intersection points P_(γ).

Knot vector V, by construction, will contain knots originating fromthree sources. The first source will be from the significant pointlocations defined by knots v_(α). The second source will be from theinternal surface knots v_(x) _(i) . Thirdly, the last source of knotsare associated with those used in the description of the parametriccurve v_(ψ) approximating the surface-surface intersection, in additionto the above source of knots, as to satisfy tolerance criteria.

Significant point knots v_(α) of types 0 and 1 define changes in whichparameterization change handling is required. These significant pointsare unique in that they either define a parametric degeneracy or thatthey characterize a change from a classified region x^(i) to thecomplementary classified region y^(i). In doing so, the points wherethey occur on surface S_(i) necessitate distinct operations that resultin equivalent geometries (surface point sets) with very differentparameterizations.

When significant point knots v_(α) of types 0 or 1 are encountered, oneof three different methods may be employed, defining the parameterchange handling algorithm. Hereinafter, for the parameter changehandling algorithm, we refer to these three methods as:

-   -   1. C⁰ points;    -   2. Embedded Extensions; and    -   3. Inserted Extraordinary Points        Each of these three methods will be defined in detail below.

One of the three methods in the parameter change handling algorithm isspecified to complete either the Pre-SSI and Post-SSI algorithms. Thedownstream operations require the completion of the parameter changehandling algorithm to successfully complete the operations defining fullexecution of the respective algorithm.

C⁰ Points

Specifying C⁰ points is equivalent to running either the Pre-SSI orPost-SSI algorithms without any intervention at this point. There are noadditional operations taken and the data in the previous stages serve asinput to the downstream stages (i.e., flow through of information).

Making the designation of C⁰ points results in C⁰ surface continuity atpoints mapped from knots v_(a) of types 0 or 1 to points on surfaceS_(i) when global reparameterization and reconstruction occurs. By notselecting one of the other two methods of the parameter change handlingalgorithm, processing the remainder of operations in the Pre-SSI orPost-SSI algorithms results in lines of C⁰ surface continuity atS_(i)(s^(i), t^(i))_(type α0) ^(i) and/or S_(i)(s^(i), t^(i))_(type α1)^(i), reflected in the global parameter space of {tilde over (Ω)}.

The realization of the C⁰ point selection will vary as per thesignificant point knot type 0 or 1. Making the designation of a C⁰ pointis on a point-by-point basis and not required as inclusive to a groupbased on designation.

In the instance where a C⁰ point of significant point knot type 0occurs, a line of C⁰ surface continuity at points mapped from knotsv_(0α) on surface S_(i) in the y^(i) direction will be produced fromdownstream global reparameterization and reconstruction operations. Thisis demonstrated in an example in FIG. 40.

In the instance where a C⁰ point of significant point knot type 1occurs, a line of C⁰ surface continuity at points mapped from knotsv_(0α) on surface S_(i) in x^(i) and y^(i) directions outwards fromradius of curvature of the original parameter space trim curves will beproduced from downstream global reparameterization and reconstructionoperations. Another way of saying this is that the curve normal vectorindicates C⁰ surface continuity of the reconstructed surface in thex^(i) and y^(i) directions. This is demonstrated in an example in FIG.41.

Embedded Extensions

Specifying Embedded Extensions is equivalent to running either thePre-SSI or Post-SSI algorithms with the same set of surface-surfaceintersection trim curves designated as input to the parameter changehandling algorithm but with additional extensions made to the individualcurves. The details of the process to create these extensions areoutlined below.

Making the designation of Embedded Extensions results in surfacecontinuity greater than C⁰ at points mapped from knots v_(α) of types 0or 1 to points on surface S_(i) when global reparameterization andreconstruction occurs. By selecting the Embedded Extensions method ofthe parameter change handling algorithm, processing the remainder ofoperations in the Pre-SSI or Post-SSI algorithms results in continuitygreater than C⁰ at S_(i)(s^(i), t^(i))_(type α0) ^(i) and/orS_(i)(s^(i), t^(i))_(type α1) ^(i), reflected in the global parameterspace of {tilde over (Ω)}.

The operations defining the Embedded Extensions selection are the samefor either significant point knot type 0 or 1, but with differingresults. Making the designation of Embedded Extensions is on apoint-by-point basis and not required as inclusive to a group based ondesignation.

In the instance where an Embedded Extensions is specified for asignificant point, the parameter space trim curveC_(PSi)(u^(i))=[s^(i)(u^(i)), t^(i)(u^(i))]^(T) is extended in aparticular fashion in the domain of the original parameter spacesΩ^(i)=[s_(min) ^(i), s_(max) ^(i)]×[t_(min) ^(i), t_(max) ^(i)]⊂

². The goal of the process is to extend the curve at its clustered endpoint C_(PSi)(u^(i) _(END)) containing arbitrary tangent derivativedC_(PSi)/du_(END) ^(i)=[ds^(i)/du_(END) ^(i), dt^(i)/du_(END) ^(i)]^(T)with a CP⁻¹ extension such that it smoothly curves to orient to a newend C_(PSi)(u_(EXT) ^(i)) whose tangent curve vector is parallel to theclassified section parameter, y^(i), at arbitrary value x^(i)=y ^(i),i.e. dy ^(i)/du_(EXT) ^(i)=0. We will refer to the extended version ofC_(PSi)(u^(i)) as C_(PSi_EXT)(u^(i)).

Spline curve extensions are commonly covered in CAGD literature and arefeatured commonly in CAD applications. The particular extension usedhere has not been documented to date for the purposes described.

The trace of the C_(PSi_EXT)(u^(i)) can be evaluated to furnish a set ofmodel space points SOP_(MS_EXT) lying on surface S_(i). These points canbe interpolated using standard techniques in CAD to furnish an extendedversion of C_(MS)(v) denoted C_(MS_EXT)(19). Similar to C_(MS)(v),C_(MS_EXT)(v) will not lie directly on surface S_(i). This is not thepurpose of creating C_(MS_EXT)(v), rather, to have the new end pointC_(MS_EXT)(v_(EXT)) tangent to an isoparametric curve of the classifiedsection parameter, y^(i)=y ^(i), S_(i)(x^(i), y ^(i)).

Having created C_(MS_EXT)(v) such that it smoothly transitions fromC_(MS)(v_(END)) to a point C_(MS_EXT)(v_(EXT)) whose tangent vectoraligns with the isoparametric curve of the classified section parameter,y^(i)=y ^(i), S_(i)(x^(i), y ^(i)), the downstream operations of globalreparameterization and reconstruction can now be processed normally. Thecurve extensions created by the above process create unique andadvantageous parametric and surface properties at S_(i)(s^(i),t^(i))_(type α0) ^(i) and/or S_(i)(s^(i), t^(i))_(type α1) ^(i) of thereconstructed surface (e.g., properties such as geometric and parametriccontinuity, parametric structure, parametric fidelity, mesh resolution,mesh aspect ratio, mesh skew, mesh taper, etc.), reflected in the newglobal parameter space of {tilde over (Ω)}.

Examples of the Embedded Extensions for the significant point knot types0 and 1 are depicted in FIG. 42 and FIG. 43, respectively, showing howthe significant point knot types affect the final objects.

Inserted Extraordinary Points

Specifying Inserted Extraordinary Points is equivalent to running eitherthe Pre-SSI or Post-SSI algorithms with the same set of surface-surfaceintersection trim curves designated as input to the parameter changehandling algorithm but with an additional trim curve created exclusivelyfor the purposes of altering the parameterization and extraordinarypoint characterization of the object. To date, the use of suchtechniques has never been documented nor utilized in any available CADapplication.

Making the designation of Inserted Extraordinary Points results insurface continuity greater than C⁰ at points mapped from knots v_(α) oftype 1 to points on surface S_(i) when global reparameterization andreconstruction occurs. By selecting the Inserted Extraordinary Pointsmethod of the parameter change handling algorithm, processing theremainder of operations in the Pre-SSI or Post-SSI algorithms results incontinuity greater than C⁰ at S_(i)(s^(i), t^(i))_(type α1) ^(i),reflected in the global parameter space of {tilde over (Ω)}.

Making the designation of Inserted Extraordinary Points is on apoint-by-point basis and not required as inclusive to a group based ondesignation.

In the instance where an Inserted Extraordinary Points is specified fora significant point, an additional parameter space trim curveC_(EPi)(r^(i))=[s^(i)(r^(i)), t^(i)(r^(i))]^(T) is created in aparticular fashion in the domain of the original parameter spacesΩ^(i)=[s_(min) ^(i), s_(max) ^(i)]×[t_(min) ^(i), t_(max) ^(i)]⊂

². The goal of the process is to establish a new curve used to modifythe parameterization of S_(i) when used in combination with thedownstream operations of global reparameterization and reconstructionprocessed normally.

C_(EPi)(r^(i))=[s^(i)(r^(i)), t^(i)(r^(i))]^(T) is constructed to be anew curve segment normal to the curve C_(PSi)(u^(i))=[s^(i)(u^(i)),t^(i)(u^(i))]^(T) (orthogonal to the curve tangent) at the specifiedsignificant point, C_(PSi)(u_(α) ^(i))=[s^(i)(u_(α) ^(i)), t^(i)(u_(α)^(i))]^(T).

The simplest construction of C_(EPi)(r^(i))=[s^(i) (r^(i)),t^(i)(r^(i))]^(T) is a line segment that extends in both the negativeand positive directions normal to the curveC_(PSi)(u^(i))=[s^(i)(u^(i)), t^(i)(u^(i))]^(T), the distance away ineach direction being an arbitrary specified distance. More complexconstructions of C_(EPi)(r^(i)) are admissible, as long as they extendin both the negative and positive directions normal to the curveC_(PSi)(u^(i)), but are not necessary.

The trace of C_(EPi)(r^(i)) can be evaluated to furnish a set of modelspace points SOP_(MS_EP) lying on surface S_(i). These points can beinterpolated using standard techniques in CAD to furnish an insertedextraordinary point model space curve denoted C_(MS_EP)(ρ). Similar toC_(MS)(v), C_(MS_EP)(ρ) will not lie directly on surface S_(i). This isnot the purpose of creating C_(MS_EP)(ρ), rather, to have the new curvenormal to an isoparametric curve of the classified section parameter,y^(i)=y ^(i), S_(i)(x^(i), y ^(i)).

Having created, C_(MS_EP)(ρ) such that it is a new curve normal to anisoparametric curve of the classified section parameter, y^(i)=y ^(i),S_(i)(x^(i), y ^(i)), the downstream operations of globalreparameterization and reconstruction can now be processed normally. Theinserted extraordinary point model space curves created by the aboveprocess create unique and advantageous parametric and surface propertiesat S_(i)(s^(i), t^(i))_(type α1) ^(i) of the reconstructed surface(e.g., properties such as geometric and parametric continuity,parametric structure, parametric fidelity, mesh resolution, mesh aspectratio, mesh skew, mesh taper, etc.), reflected in the new globalparameter space of {tilde over (Ω)}.

An example of the Inserted Extraordinary Point technique is depicted inFIG. 44.

The parameter change handling algorithm modifies the input informationcreated by both Pre-SSI and Post-SSI algorithms to create unique andadvantageous parametric and surface properties at S_(i)(s^(i),t^(i))_(type α0) ^(i) and/or S_(i)(s^(i), t^(i))_(type α1) ^(i) of thereconstructed surface (e.g., properties such as geometric and parametriccontinuity, parametric structure, parametric fidelity, mesh resolution,mesh aspect ratio, mesh skew, mesh taper, etc.), reflected in the newglobal parameter space of {tilde over (Ω)}. Multiple schemes areavailable, distinguished by the resulting continuity, complexity, andstructure, as detailed by the C⁰ Points, Embedded Extensions, or theInserted Extraordinary Points methods. As such, the parameter changehandling algorithm can be used to reparametrize a surface even without asurface-surface intersection or Boolean operation specified. By invokingthe Inserted Extraordinary Point technique, untrimmed objects mayexperience the benefits of the processes herein at selectedextraordinary points and/or degenerate points. FIG. 45 depicts such aninstance in which the surface of a sphere is modified from a quadballconfiguration to that of a standard single patch configuration withdegenerate points at the poles.

Graphics Device

In one set of embodiments, a graphics device including a graphicsprocessing unit (GPU) (or a plurality of GPUs) and memory may beconfigured to perform a method (e.g., any of the method embodimentsdescribed herein, or, any combination of the method embodimentsdescribed herein, or any subset of any of the method embodimentsdescribed herein, or, any combination of such subsets). A GPU programstored in the memory may be executed by the GPU, in order to performsaid method. The graphics device may enable said method to operate inreal time. The graphics device may be configured for coupling to orinterfacing with a computer system. The graphics device may becontrolled by host software (e.g., CAD software) executing on thecomputer system.

Some embodiments may be realized in any of the following various forms.

1. A method comprising:

-   -   receiving first and second geometric input data that        respectively specify first and second input surfaces in a model        space;    -   creating first and second geometric output data that        respectively specify first and second output surface patches,        wherein the first and second geometric output data are created        so that:    -   a boundary portion of the first output surface patch meets a        boundary portion of the second output surface patch in a        continuous fashion along at least a portion of a model space        curve,    -   wherein the model space curve approximates a geometric        intersection of the first input surface and the second input        surface,    -   wherein the first output surface patch geometrically        approximates a subsurface of the first input surface,    -   wherein the second output surface patch geometrically        approximates a subsurface of the second input surface.

2. The method of claim 1,

-   -   wherein the first input surface includes a first tensor product        spline surface, wherein the first geometric input data includes        a first pair of knot vectors and a first set of surface control        points,    -   wherein the second input surface includes a second tensor        product spline surface, wherein the second geometric input data        includes a second pair of knot vectors and a second set of        surface control points.

2B. The method of claim 2, wherein the first and second tensor productspline surfaces are NURBS surfaces (or T-Spline surfaces).

3. The method of claim 1, wherein the first output surface patchgeometrically approximates the subsurface of the first input surface towithin model tolerance, wherein the second output surface patchgeometrically approximates the subsurface of the second input surface towithin the model tolerance

4. The method of claim 1, wherein the boundary portion of the firstoutput surface patch corresponds to a boundary isocurve of the firstoutput surface patch, wherein the boundary portion of the second outputsurface patch corresponds to a boundary isocurve of the second outputsurface patch.

5. The method of claim 1, wherein said receiving and said creating areperformed as part of a Boolean operation in a CAD software system.

6. The method of claim 1, further comprising:

-   -   numerically computing a set of intersection points that at least        approximately reside on the geometric intersection of the first        input surface and the second input surface;    -   computing geometric data (e.g., knot vector and curve control        points) that specify the model space curve, based on the set of        intersection points; and    -   storing the geometric data that specify the model space curve.

7. The method of claim 1, further comprising:

-   -   traversing a boundary representation to obtain the first and        second geometric input data, wherein the boundary representation        models topological and geometric structure of an object.

8. The method of claim 7, further comprising:

-   -   updating the boundary representation in response to said        creating the first and second geometric output data.

9. The method of claim 8, further comprising:

-   -   after having performed said update of the boundary        representation, executing an engineering analysis (e.g., a        physics-based analysis) based on the boundary representation, to        obtain data predicting physical behavior of the object. (The        predictive data may be used in various ways, e.g., to: generate        display output to a user, for visualization of the physical        behavior; calculate and output a set of performance limits for a        manufactured realization of the object; identify locations of        likely fault(s) in a manufactured realization of the object;        select material(s) to be used for manufacture of the object;        direct a process of manufacturing the object; direct automatic        changes to geometry of the boundary representation; etc.)

10. The method of claim 9, wherein said engineering analysis includes(or is) an isogeometric analysis.

11. The method of claim 8, further comprising:

-   -   after having performed said update of the boundary        representation, manufacturing (or directing a process of        manufacturing) the object based on the boundary representation.

12. The method of claim 8, further comprising:

-   -   after having performed said update of the boundary        representation, generating an image of the object based on the        boundary representation and displaying the image.

13. The method of claim 8, further comprising:

-   -   after having performed said update of the boundary        representation, generating a sequence of animation images based        on the boundary representation and displaying the sequence of        animation images.

14. The method of claim 1, further comprising:

-   -   any subset of the features, elements and embodiments described        in the specification and Figures.

15. The method of claim 1, wherein said creating the first geometricoutput data includes:

-   -   determining a first set of isocurve data specifying a first set        of isocurves of the first input surface on a region within a        parametric domain of the first input surface;    -   dividing the first set of isocurves at respective locations        based on the model space curve, to obtain a second set of        isocurve data specifying sub-isocurves of the respective        isocurves;    -   computing surface control points for the first output surface        patch based on control points for said portion of the model        space curve and a proper subset of the second set of isocurve        data.

16. The method of claim 15, wherein said sub-isocurves arereparametrized to a common parametric interval and cross-refined toachieve a common knot vector prior to said computing the surface controlpoints for the first output surface patch.

17. The method of claim 1,

-   -   wherein said computing the surface control points for the first        output surface patch comprises solving one or more linear        systems of equations that relate said surface control points to        the control points for said portion of the model space curve and        said at least a portion of the first set of isocurve data.

18. A method for designing an object (e.g., a part for an automobile, anaircraft, a submarine, a boat, an electronic device, etc.), the methodcomprising:

-   -   reconstructing trimmed surfaces in a boundary representation        model of the object, so that the reconstructed trimmed surfaces        meet in a C⁰ continuous fashion along their respective trim        boundaries; and    -   updating the boundary representation model based on the        reconstructed trimmed surfaces.

19. A method for designing an object, the method comprising:

-   -   constructing trimmed surfaces that meet in a C⁰ continuous        fashion along their respective trim boundaries; and    -   updating a boundary representation model of the object based on        the constructed trimmed surfaces.

20. A method comprising:

-   -   receiving first and second geometric input data that        respectively specify first and second input surfaces in a model        space;    -   creating first geometric output data that specify a first set of        output surface patches;    -   creating second geometric output data that specify a second set        of output surface patches;    -   wherein the first and second geometric output data are created        so that:    -   a boundary portion of a union of images of the first set of        output surface patches meets a boundary portion of a union of        images of the second set of output surface patches in a        continuous fashion along at least a portion of a model space        curve,    -   wherein the model space curve approximates a geometric        intersection of the first input surface and the second input        surface,    -   wherein the union of the images of the first set of output        surface patches geometrically approximates a subsurface of the        first input surface,    -   wherein the union of the images of the second set of output        surface patches geometrically approximates a subsurface of the        second input surface.

21. The method of claim 20, further comprising any subset of thefeatures recited in claims 1-19.

22. The method of claim 20,

-   -   wherein the first input surface includes a first tensor product        spline surface, wherein the first geometric input data includes        a first pair of knot vectors and a first set of surface control        points,    -   wherein the second input surface includes a second tensor        product spline surface, wherein the second geometric input data        includes a second pair of knot vectors and a second set of        surface control points.

23. The method of claim 20,

-   -   wherein the image of each of the first set of output surface        patches geometrically approximates a corresponding portion of        the first input surface to within model tolerance,    -   wherein the image of each of the second set of output surface        patches geometrically approximates a corresponding portion of        the second input surface to within the model tolerance.

24. The method of claim 20, further comprising:

-   -   traversing a boundary representation to obtain the first and        second geometric input data, wherein the boundary representation        models topological and geometric structure of an object; and/or    -   updating the boundary representation in response to said        creating the first and second geometric output data.

25. The method of claim 24, further comprising:

-   -   performing refinement on the first input surface, the second        input surface and the model space curve to obtain:    -   a one-to-one correspondence between portions of the model space        curve and knot intervals of the first input surface; and    -   a one-to-one correspondence between portions of the model space        curve and knot intervals of the second input surface;    -   for each of the portions of the model space curve, performing        operations including:    -   creating a corresponding one of the first set of output surface        patches on the corresponding knot interval of the first input        surface;    -   creating a corresponding one of the second set of output surface        patches on the corresponding knot interval of the second input        surface,    -   wherein a boundary portion of the created output surface patch        of the first set meets a boundary portion of the created output        surface patch of the second set in a continuous fashion along a        corresponding portion of the model space curve.

26. The method of claim 25, wherein said performing refinement includes:

-   -   identifying significant points on a trajectory of a first        parametric trim curve that resides in a parametric domain of the        first input surface;    -   refining the first input surface based on the identified        significant points;    -   refining the second input surface based on the identified        significant points.

27. The method of claim 26, wherein the first geometric input datadefine a first spline mapping for the first input surface, the methodfurther comprising:

-   -   numerically computing a set of intersection points in the        parametric domain of the first input surface, wherein images of        the intersection points under the first spline map at least        approximately reside on the geometric intersection of the first        input surface and the second input surface; and    -   computing and storing geometric data (e.g., knot vector and        curve control points) that specify the first parametric trim        curve, based on the set of intersection points.

28. The method of claim 25, wherein said performing refinement includes:

-   -   identifying one or more interior surface knots of the first        input surface;    -   refining the first input surface at the one or more interior        knots; and    -   refining the second input surface based on the one or more        interior knots of the first input surface.

29. The method of claim 25, wherein said performing refinement includes:

-   -   identifying one or more knots of the model space curve;    -   refining the first input surface based on the one or more knots        of the model space curve;    -   refining the second input surface based on the one or more knots        of the model space curve.

30. The method of claim 25, wherein said creating the correspondingoutput surface patch of the first set includes:

-   -   reparametrizing first and second boundary isocurves of the        output surface patch to achieve parametric agreement with the        corresponding portion of the model space curve.

31. The method of claim 25, wherein said creating the correspondingoutput surface patch of the first set includes:

-   -   determining a first set of isocurve data specifying a first set        of isocurves of the first input surface on a region within a        parametric domain of the first input surface;    -   dividing the first set of isocurves at respective locations        based on the model space curve, to obtain a second set of        isocurve data specifying sub-isocurves of the respective        isocurves;    -   computing surface control points for the output surface patch        based on control points for the corresponding portion of the        model space curve and a proper subset of the second set of        isocurve data.

32. A computer-implemented method for modifying an object comprising atleast two surfaces, to create trimmed surfaces that meet in a continuousfashion, the method comprising:

-   -   receiving and storing input data including:    -   first and second spline surfaces;    -   a model space trim curve that approximates an intersection of        the first and second spline surfaces in a model space;    -   extracting first and second initial surface patches respectively        from the first and second spline surfaces based on the input        data;    -   extracting a portion of the model space trim curve;    -   constructing a first output patch based on curve control points        of said portion of the model space trim curve and sampled        sub-isocurve data from the first initial surface patch;    -   constructing a second output patch based on the curve control        points of said portion of the model space trim curve and sampled        sub-isocurve data from the second initial surface patch;    -   wherein the first and second output patches are constructed so        that a boundary portion of the first output patch meets a        boundary portion of the second output patch along said portion        of the model space trim curve.

33. The method of claim 32, wherein the first and second output patchesare constructed so that, for each of the first and second outputpatches:

-   -   the image of said output patch is within model tolerance of a        corresponding portion of the corresponding initial surface        patch.

34. The method of claim 32, wherein the input data also includes firstand second parameter space trim curves that correspond respectively tothe first and second spline surfaces.

35. The method of claim 32, wherein at least one of the first and secondoutput surface patches is of Bézier form in at least one parametricdirection.

36. The method of claim 32, wherein said portion of the model spacecurve is of Bézier form.

37. A method for reconstructing at least two tensor product splinesurfaces to provide that the surfaces meet with C⁰ continuity along atleast a portion of a model space curve, wherein said at least two tensorproduct spline surfaces each map from a corresponding 2D parameter spaceinto a common model space, and each has a corresponding trim curve thatresides in the corresponding 2D parameter space, wherein the model spacecurve resides in the model space and approximates a geometricintersection of said at least two tensor product spline surfaces, themethod comprising:

-   -   for each of said at least two tensor product spline surfaces,        performing operations including:    -   generating a set of surface control points for a surface patch        from at least a portion of the tensor product spline surface,        wherein a portion of the trim curve of the tensor product spline        surface traverses a 2D domain of the surface patch from a first        boundary to a second boundary opposite the first boundary,        wherein the first boundary corresponds to a minimal value of a        selected coordinate of the 2D domain, wherein the second        boundary corresponds to a maximal value of the selected        coordinate of the 2D domain;    -   for each of at least three values of the selected coordinate,        dividing a corresponding isoparametric curve of the surface        patch into a corresponding pair of sub-isoparametric curves at a        corresponding location determined based on the model space        curve, wherein said at least three values of the selected        coordinate include the minimal value, the maximal value and at        least one intermediate value;    -   generating a set of surface control points for a first output        patch based on control point data including:    -   (a) for each of the at least one intermediate value, a        corresponding set of interior curve control points of a first        sub-isoparametric curve of the sub-isoparametric curve pair        corresponding to the intermediate value;    -   (b) for the minimal value of the selected coordinate, a first        set of curve control points of a first sub-isoparametric curve        SI_(1,min) of the sub-isoparametric curve pair corresponding to        the minimal value;    -   (c) for the maximal value of the selected coordinate, a second        set of curve control points of a first sub-isoparametric curve        SI_(1,max) of the sub-isoparametric curve pair corresponding to        the maximal value;    -   (d) curve control points for said portion of the model space        curve.

38. The method of claim 37, wherein said control point data alsoincludes: (e) curve control points for an isoparametric curve of thesurface patch along a third boundary of the 2D domain, wherein the thirdboundary corresponds to either a maximal value or a minimal value of acomplementary coordinate of the 2D domain.

39. The method of claim 37, wherein said at least three values of theselected coordinate include at least four values of the selectedcoordinate.

40. The method of claim 37, wherein the maximal value and/or the minimalvalue of the complementary coordinate of the 2D domain is (are) selectedbased on user input.

41. The method of claim 37, wherein the first set of curve controlpoints does not include a curve control point of the sub-isoparametriccurve SI_(1,min) that is closest to the model space curve, wherein thesecond set of curve control points does not include a curve controlpoint of the sub-isoparametric curve SI_(1,max) that is closest to themodel space curve.

42. The method of claim 37, wherein the first output patch is generatedso that a boundary isoparametric curve of the first output patch mapsonto the portion of the model space curve.

43. The method of claim 37, further comprising:

-   -   traversing a boundary representation object model to identify at        least two topological faces that meet along a topological edge,        wherein the boundary representation object model includes        geometric data for each of the topological faces and the        topological edge, wherein each tensor product spline surface and        its corresponding trim curve are included in the geometric data        associated with a corresponding one of the topological faces,        wherein the model space curve is included in the geometric data        associated with the topological edge.

44. The method of claim 43, further comprising:

-   -   for each of said at least two topological faces, generate a        corresponding set of output geometry using at least the set of        surface control points of the first output patch, and update the        geometry associated with the topological face based on the        corresponding set of output geometry.

45. The method of claim 44, further comprising, after said updating thegeometry of each of said at least two topological faces:

-   -   performing an isogeometric finite element analysis based on the        boundary representation object model, to obtain analysis data        characterizing physical behavior of the boundary representation        object model; and    -   outputting at least a subset of the analysis data for        visualization via a display device.

46. The method of claim 44, further comprising, after said updating thegeometry of each of said at least two topological faces:

-   -   performing a finite element analysis on a polygonal mesh        generated from the boundary representation object model, to        obtain analysis data characterizing physical behavior of the        boundary representation object model; and    -   outputting at least a subset of the analysis data for        visualization via a display device.

47. The method of claim 44, further comprising, after said updating thegeometry of each of said at least two topological faces:

-   -   manufacturing (or directing the manufacture) of a physical        object based on the boundary representation object model.

48. The method of claim 47, wherein said manufacturing includesdirecting an additive manufacturing device to manufacture the physicalobject based on the boundary representation object model.

49. The method of claim 44, further comprising, after said updating thegeometry of each of said at least two topological faces:

-   -   generating a 2D image based on the boundary representation        object model, relative to a set of viewing parameters (such as a        model space viewpoint, a model space viewing direction, a view        projection); and    -   displaying the 2D image via a display device.

50. The method of claim 44, further comprising, after said updating thegeometry of each of said at least two topological faces:

-   -   generating a sequence of animation images based on the boundary        representation object model; and    -   displaying the sequence of animation images via a display        device, to animate the object model.

51. The method of claim 44, further comprising, after said updating thegeometry of each of said at least two topological faces:

-   -   generating a three dimensional mesh based on the boundary        representation object model;    -   performing one or more of the following processes on the three        dimensional mesh:        -   executing an engineering analysis on the three dimensional            mesh;        -   manufacturing a physical object based on the three            dimensional mesh;        -   displaying an image based on the three dimensional mesh;    -   displaying a sequence of animation images based on the three        dimensional mesh.

52. The method of claim 43, wherein said at least two tensor productspline surfaces comprise three or more tensor product spline surfaces,wherein said boundary representation object model represents an objectthat is not a 2-manifold.

53. The method of claim 43, wherein said boundary representation objectmodel represents a solid object whose topological boundary represents a2-manifold.

54. The method of claim 37, wherein said operations also include:

-   -   prior to said dividing of the isoparametric curves,        reparametrizing the selected coordinate of the surface patch to        achieve parametric agreement with the portion of the model space        curve.

55. The method of claim 37, wherein said generating the set of surfacecontrol points for the first output patch includes solving a linearsystem that relates interior ones of the surface control points of thefirst output patch to (a).

56. The method of claim 37, further comprising:

-   -   for each of said at least two tensor product spline surfaces,        identifying significant points of the corresponding trim curve,        and refining the tensor product spline surface based on the        identified significant points;    -   transferring knot information between the model space curve and        each of said at least two tensor product spline surfaces;    -   refining the model space curve and said at least two tensor        product spline surfaces based on the transferred knot        information, wherein said portion of the model space curve is a        Bézier portion of the model space curve, wherein, for each of        said at least two tensor product spline surfaces, the        corresponding surface patch is of Bézier form in at least the        corresponding selected coordinate tensor product.

57. The method of claim 37, wherein said portion of the model spacecurve is a Bézier portion, wherein the number of said at least threevalues of the selected coordinate is equal to the degree of said Bézierportion plus one.

-   -   Bivariate Spline Reparameterization

1.0 A method for reparametrizing a bivariate parametric surface using abivariate spline function, the method comprising:

-   -   receiving and storing input data comprising:    -   a first surface knot vector of the bivariate parametric surface        to be reparametrized;    -   a curve knot vector of a trim curve to which the bivariate        parametric surface will be reparametrized;    -   a target surface knot vector for a reparameterization of the        bivariate parametric surface;    -   a field of height values for the bivariate spline function;    -   determining a control net for the bivariate spline function        based on the input data, wherein bivariate spline function and        the determined control net specify the reparameterization of the        bivariate spline surface;    -   reparametrizing a set of isocurves of the bivariate parametric        surface using the bivariate spline function and the determined        control net;    -   computing and storing a bivariate output surface based on the        reparametrized set of isocurves.

1.1 The method of claim 1.0, wherein said receiving, said determining,said reparametrizing and said computing are performed for each of aplurality of surfaces involved in a Boolean operation in order to obtaina watertight boundary representation of an object.

1.2 The method of claim 1.1, further comprising performing one or moreof the following operations on the watertight boundary representation:

-   -   executing a computational engineering analysis on the watertight        boundary representation to predict physical behavior of the        object;

3D printing the object based on the watertight boundary representation;

-   -   generating and/or displaying and/or storing a 3D graphical        rendering of the object based on the watertight boundary        representation;    -   directing a numerically controlled machine to manufacture the        object based on the watertight boundary representation.

2.0 A method for reparametrizing a bivariate parametric surface using abivariate spline function, the method comprising:

-   -   receiving input data comprising:    -   a first surface knot vector of the bivariate parametric surface        to be reparametrized;    -   a curve knot vector of a trim curve to which the bivariate        parametric surface will be reparametrized;    -   a target surface knot vector for a reparameterization of the        bivariate parametric surface;    -   one or more curvature tolerances;    -   determine a control net for the bivariate spline function based        on the input data, wherein the bivariate spline function and the        determined control net specify the reparameterization of the        bivariate spline function;    -   reparametrizing a set of isocurves of the bivariate parametric        surface using the bivariate spline function and the determined        control net;    -   computing and storing a bivariate output surface based on the        reparametrized set of isocurves.    -   Pre-SSI Algorithm (Correct by Construction)

3.0 A method for determining surface-surface intersection data tosupport a Watertight Boolean CAD operation on a set of two or moresurfaces, the method comprising:

-   -   determining a set of division points on the surface-surface        intersection, wherein said determining includes:    -   for each surface, determining a first set of points comprising        significant points and internal surface knots by cluster;    -   exchanging the first sets between surfaces to obtain for each        surface a corresponding second set of points that is a superset        of the corresponding first set;    -   merge the model space images of the second sets into a combined        model space point set that is ordered according to topology of        the SSI.

3.1 The method of claim 3.1, further comprising:

-   -   performing a watertight Boolean CAD operation based on data        including at least the set of division points, to obtain a        watertight boundary representation of an object.

3.2 The method of claim 3.1, wherein said performing the watertightBoolean CAD operation includes:

-   -   for each of the surfaces, reparametrizing a set of isocurves of        the surface using a bivariate spline function.

3.3 The method of claim 3.1, further comprising performing one or moreof the following operations on the watertight boundary representation:

-   -   executing a computational engineering analysis on the watertight        boundary representation to predict physical behavior of the        object;

3D printing the object based on the watertight boundary representation;

-   -   generating and/or displaying and/or storing a 3D graphical        rendering of the object based on the watertight boundary        representation;    -   directing a numerically controlled machine to manufacture the        object based on the watertight boundary representation.

4.0 A method for parameter change handling, to support a process ofwatertight geometry construction, the method comprising:

-   -   for a given point of a plurality of points, selecting a        parameter change handling (PCH) algorithm, wherein the points        are points on a geometric intersection between input surfaces,        where PCH algorithm is selected from a set of available PCH        algorithms including a null algorithm, an embedded extensions        algorithm and an extraordinary point insertion algorithm;    -   performing the selected PCH algorithm on the given point to        obtain a tensor product spline output surface closely        approximating the geometry of a tensor product spline input        surface.

4.0.1 The method of claim 4.0, wherein the tensor product spline outputsurface is reparametrized with specific properties at and around thegiven point.

4.1 The method of claim 4.0,

-   -   wherein the null algorithm is a pass through algorithm that does        nothing;    -   wherein the embedded extensions algorithm performs tangent        extension without introducing extraordinary points;    -   wherein the extraordinary point insertion algorithm introduces        an extra (fictitious) curve that is orthogonal to the parameter        space trim curve at the given point (e.g., acknowledging the        mesh the user is looking for).

4.2 The method of claim 4.0, wherein the selected PCH algorithm createscurves for reparameterization and geometry construction (orreconstruction) at the given point.

4.3 The method of claim 4.0, wherein the selected PCH algorithm ensurescurve continuity when the given point is an inflection point.

4.4 The method of claim 4.0, wherein the selected PCH algorithm changesthe parameterization of a tensor product spline surface closelyapproximating the geometry of the tensor product spline surface inputaround the given point.

4.5 The method of claim 4.0, wherein the selected PCH algorithm createsunique curves for reparameterization and/or reconstruction that createunique reparameterization and/or continuity and/or targeted surfacestructure at inflection points.

4.6 The method of claim 4.0, further comprising:

-   -   generating a watertight boundary representation of an object        based on specific reparameterization properties desired at        computed inflection points; and    -   performing one or more of the following operations on the        watertight boundary representation:    -   executing a computational engineering analysis on the watertight        boundary representation to predict physical behavior of the        object;

3D printing the object based on the watertight boundary representation;

-   -   generating and/or displaying and/or storing a 3D graphical        rendering of the object based on the watertight boundary        representation;    -   directing a numerically controlled machine to manufacture the        object based on the watertight boundary representation.

5.0 A method for parameter change handling (PCH), to support a processof watertight geometry construction, the method comprising:

-   -   for a given point of a plurality of points including        extraordinary and/or degenerate points on a geometric input        surface;    -   performing an extraordinary point insertion type of PCH        algorithm on the given point to obtain a tensor product spline        output surface closely approximating the geometry of a tensor        product spline input surface.

5.0.1 The method of claim 5.0, wherein the tensor product spline outputsurface is reparametrized with specific properties (e.g., anycombination of the following properties: geometric and parametriccontinuity, parametric structure, parametric fidelity, mesh resolution,mesh aspect ratio, mesh skew, mesh taper, etc.) at and around the givenpoint.

5.1 The method of claim 5.0,

-   -   wherein the extraordinary point insertion algorithm introduces        an extra (fictitious) curve that is orthogonal to a parameter        space trim curve at the given point (e.g., acknowledging the        mesh the user is looking for).

5.2 The method of claim 5.0,

-   -   wherein the method has no dependence on surface-surface        intersections or Boolean operations of solid model boundary        representations;    -   wherein the input is a single surface containing all of the        requisite information to perform the operations listed.

6.0. A method comprising:

-   -   constructing and storing a boundary representation model of an        object, wherein the boundary representation model includes        output surfaces, wherein an isocurve of a first of the output        surfaces meets an isocurve of a second of the output surfaces in        a continuous fashion.

6.1 The method of claim 6.0, wherein the first output surface and thesecond output surface are non-planar surfaces.

6.2 The method of claim 6.0, wherein said constructing the boundaryrepresentation model of the object is performed as part of a Boolean CADoperation on a set of two or more input surfaces, wherein each of theoutput surfaces corresponds to a respective one of the input surfaces.

6.2.1 The method of claim 6.2, wherein, for each pair of the inputsurfaces that geometrically intersect, the corresponding pair of outputsurfaces meet in a continuous fashion along respective isocurves.

6.3 The method of claim 6.0, further comprising:

-   -   performing a computer-based engineering analysis on the object        based on the boundary representation model (without any need for        gap-remediation on the boundary representation model), wherein        the computer-based engineering analysis calculates data        representing physical behavior of the object; and    -   storing and/or displaying the data representing the physical        behavior of the object.

6.4 The method of claim 6.3, wherein the data represents one or more of:

-   -   a predicted location of a fault in the object;    -   a decision on whether the object will tolerate (or survive or        endure) a user specified profile of applied force (or pressure        or stress or thermal stimulus or radiation stimulus or        electromagnetic stimulus);    -   a predicted thermal profile (or stress profile) of an engine        under operating conditions.

6.5 The method of claim 6.0, further comprising:

-   -   directing one or more numerically controlled machines (without        any need for gap-remediation on the boundary representation        model) to manufacture the object based on the boundary        representation model, wherein physical surfaces of the        manufactured object meet in a continuous fashion.

6.5.1 The method of claim 6.5, wherein the numerically controlledmachines include one or more of the following: a numerically controlledmill, a numerically controlled lathe, a numerically controlled plasmacutter, a numerically controlled electric discharge machine, anumerically controlled fluid jet cutter, a numerically controlled drill,a numerically controlled router, a numerically controlled bendingmachine.

6.6 The method of claim 6.0, further comprising:

-   -   employing a 3D graphics rendering engine to generate a 3D        graphical model of the object based on the boundary        representation model (without any need for gap remediation on        the boundary representation model); and    -   storing and/or displaying the 3D graphical model of the object        (e.g., as part of a 3D animation or movie).

6.7 The method of claim 6.0, further comprising:

-   -   performing 3D scan on the boundary representation model to        convert the boundary representation model into a data file for        output to a 3D printer (without any need for gap remediation on        the boundary representation model);    -   transferring the data file to a 3D printer in order to print a        3D physical realization of the object.

6.8 The method of claim 6.0, further comprising:

-   -   manufacturing a portion of a body (e.g., a hood or side panel or        roof section) of an automobile based on the boundary        representation model of the object.

6.9 The method of claim 6.0, further comprising:

-   -   manufacturing a portion of a body of a boat (or submarine) based        on the boundary representation model of the object.

6.10. The method of claim 6.0, further comprising:

-   -   downloading each of the output surfaces to a corresponding        robotic manufacturing device; and    -   directing the robotic manufacturing devices to manufacture the        respective output surfaces; and    -   assembling the manufactured output surfaces to form a composite        physical object.

A method for constructing a model of a physical object, wherein thephysical object is configured to be aligned with other physical objects,the method comprising any of the methods described herein in theDetailed Description as well as any of the methods/medium operations inthe claims section below.

A method for constructing models of two physical objects, wherein thetwo physical object are configured to be aligned with each other alongat least one edge shared by the two physical objects, the methodcomprising any of the methods described herein in the DetailedDescription as well as any of the methods/medium operations in theclaims section below.

A method for constructing models of two image objects to be displayed ona display, wherein the two image objects are configured to be alignedwith each other along at least one edge shared by the two image objects,the method comprising any of the methods described herein in theDetailed Description as well as any of the methods/medium operations inthe claims section below.

A method for constructing a model of an image object be displayed on adisplay, wherein the image object is configured to be aligned with atleast one other image object along at least one edge shared by the imageobject and the at least one other image object, the method comprisingany of the methods described herein in the Detailed Description as wellas any of the methods/medium operations in the claims section below.

A method for displaying an image of an object on a display device, themethod comprising:

-   -   constructing a model of a first portion of the image object be        displayed on the display device, wherein the first portion of        the image object is configured to be aligned with at least one        second portion of the image object along at least one edge        shared by the first portion and the at least one second portion,        wherein said constructing comprises any of the methods described        herein in the Detailed Description as well as any of the        methods/medium operations in the claims section below; and    -   displaying the image object on the display device based on the        constructed model.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

What is claimed is:
 1. A computer-implemented method for modifying acomputer-aided design (CAD) model of a tangible object, the methodcomprising: performing, by the computer: storing geometric input datadescribing first and second input surfaces associated with the CADmodel, respectively, wherein the first input surface is described in afirst parameter space domain, and wherein the second input surface isdescribed in a second parameter space domain; storing a model space trimcurve associated with the first and second input surfaces, wherein themodel space trim curve is a curve that represents a geometricintersection of the first and second input surfaces; constructing amodified CAD model of the tangible object comprising first and secondoutput surfaces, wherein the modified CAD model is constructed based onthe first and second input surfaces and the model space trim curve,wherein the modified CAD model exhibits increased inter-surfacecontinuity between the first and second output surfaces relative to thefirst and second input surfaces, and wherein the first and second outputsurfaces are described in a third parameter space domain; and storingthe modified CAD model in memory.
 2. The method of claim 1, wherein atleast a portion of the boundary of each of the first and second outputsurfaces coincides with the model space trim curve.
 3. The method ofclaim 1, wherein constructing the modified CAD model based on the firstand second input surfaces and the model space trim curve comprisesreparametrizing the first and second surfaces into the third parameterspace domain based on the model space trim curve.
 4. The method of claim3, wherein the model space trim curve comprises sequential segments ofisocurves in the third parameter space domain.
 5. The method of claim 1,wherein the model space trim curve comprises a knot vector and isocurvesampling points in the third parameter space domain, wherein theisocurve sampling points are used to sample isocurves of the first andsecond input surfaces.
 6. The method of claim 5, wherein the isocurvesampling points comprise a set of Greville points in the third parameterspace domain.
 7. The method of claim 1, wherein the first and secondoutput surfaces are within a predetermined or user-definable errortolerance of the first and second input surfaces, respectively.
 8. Themethod of claim 1, wherein the model space trim curve is received as anoutput from a Boolean operation performed on the first and second inputsurfaces in a computer-aided design (CAD) system.
 9. The method of claim1, the method further comprising: constructing the model space trimcurve based on a geometric intersection of the first and second inputsurfaces.
 10. The method of claim 9, the method further comprising:identifying and classifying segments of the model space trim curve basedon characteristic points of the geometric intersection of the first andsecond input surfaces, wherein each identified segment comprises anisocurve in the third parameter space domain, wherein saidclassification determines a parameter in the third parameter spacedomain that has a constant value along the isocurve.
 11. The method ofclaim 1, wherein the first and second input surfaces and the first andsecond output surfaces are non-uniform rational basis splines (NURBS).12. The method of claim 1, wherein said modifying is performed as partof a Boolean operation in a computer-aided design (CAD) software system.13. The method of claim 1, the method further comprising: executing anengineering analysis on the modified CAD model to obtain data predictingphysical behavior of an object described by the CAD model.
 14. Themethod of claim 13, wherein the engineering analysis comprises anisogeometric analysis.
 15. The method of claim 1, the method furthercomprising: directing a process of manufacturing an object described bythe modified CAD model.
 16. The method of claim 1, the method furthercomprising: generating an image of an object based on the modified CADmodel.
 17. The method of claim 1, the method further comprising:generating a sequence of animation images based on the modified CADmodel and displaying the sequence of animation images.
 18. The method ofclaim 1, wherein the first and second input surfaces have been trimmedbased on the geometric intersection of the first and second inputsurfaces, wherein the first and second input surfaces each comprise arespective trim curve boundary, and wherein the model space trim curveis determined based at least in part on the respective trim curveboundaries of the first and second input surfaces.
 19. A non-transitorycomputer-readable memory medium comprising program instructions which,when executed by a processor, are configured to modify a computer-aideddesign (CAD) model by causing the processor to: store geometric inputdata describing first and second input surfaces associated with the CADmodel, respectively, wherein the first input surface is described in afirst parameter space domain, and wherein the second input surface isdescribed in a second parameter space domain; store a model space trimcurve associated with the first and second input surfaces, wherein themodel space trim curve is a curve that represents a geometricintersection of the first and second input surfaces; construct amodified CAD model comprising first and second output surfaces, whereinthe modified CAD model is constructed based on the first and secondinput surfaces and the model space trim curve, wherein the modified CADmodel exhibits increased inter-surface continuity between the first andsecond output surfaces relative to the first and second input surfaces,and wherein the first and second output surface are described in a thirdparameter space domain; and store the modified CAD model in thecomputer-readable memory medium.
 20. The non-transitorycomputer-readable memory medium of claim 19, wherein at least a portionof the boundary of each of the first and second output surfacescoincides with the model space trim curve.
 21. The non-transitorycomputer-readable memory medium of claim 19, wherein constructing themodified CAD model based on the first and second input surfaces and themodel space trim curve comprises reparametrizing the first and secondsurfaces into the third parameter space domain based on the model spacetrim curve.
 22. The non-transitory computer-readable memory medium ofclaim 19, wherein the model space trim curve comprises sequentialsegments of isocurves in the third parameter space domain of themodified CAD model.
 23. The non-transitory computer-readable memorymedium of claim 19, wherein the model space trim curve comprises a knotvector and isocurve sampling points in the third parameter space domain,wherein the isocurve sampling points are used to sample isocurves of thefirst and second input surfaces.
 24. The non-transitorycomputer-readable memory medium of claim 23, wherein the isocurvesampling points comprise a set of Greville points in the third parameterspace domain.
 25. The non-transitory computer-readable memory medium ofclaim 19, wherein the first and second output surfaces are within apredetermined or user-definable error tolerance of the first and secondinput surfaces, respectively.
 26. The non-transitory computer-readablememory medium of claim 19, wherein the model space trim curve isreceived as an output from a Boolean operation performed on the firstand second input surfaces in a computer-aided design (CAD) system. 27.The non-transitory computer-readable memory medium of claim 19, whereinthe program instructions are further executable by the processor tocause the processor to: construct the model space trim curve based on ageometric intersection of the first and second input surfaces andcharacteristic points of the geometric intersection.
 28. Thenon-transitory computer-readable memory medium of claim 27, wherein theprogram instructions are further executable by the processor to causethe processor to: identify and classify segments of the model space trimcurve based on the characteristic points, wherein each identifiedsegment comprises an isocurve in the third parameter space domain of themodified CAD model, wherein said classification determines a parameterin the third parameter space domain that has a constant value along theisocurve.
 29. The non-transitory computer-readable memory medium ofclaim 19, wherein the first and second input surfaces and the first andsecond output surfaces are non-uniform rational basis splines (NURBS).30. The non-transitory computer-readable memory medium of claim 19,wherein said modifying is performed as part of a Boolean operation in acomputer-aided design (CAD) software system.
 31. The non-transitorycomputer-readable memory medium of claim 19, wherein the programinstructions are further executable by the processor to cause theprocessor to: execute an engineering analysis on the modified CAD modelto obtain data predicting physical behavior of an object described bythe CAD model.
 32. The non-transitory computer-readable memory medium ofclaim 31, wherein the engineering analysis comprises an isogeometricanalysis.
 33. The non-transitory computer-readable memory medium ofclaim 19, wherein the program instructions are further executable by theprocessor to cause the processor to perform one of: directing a processof manufacturing an object described by the modified CAD model;generating an image of an object based on the modified CAD model; orgenerating a sequence of animation images based on the modified CADmodel and displaying the sequence of animation images.
 34. Thenon-transitory computer-readable memory medium of claim 19, wherein thefirst and second input surfaces have been trimmed based on the geometricintersection of the first and second input surfaces, wherein the firstand second input surfaces each comprise a respective trim curveboundary, and wherein the model space trim curve is determined based atleast in part on the respective trim curve boundaries of the first andsecond input surfaces.
 35. A non-transitory computer-readable memorymedium comprising program instructions which, when executed by aprocessor, are configured to modify a computer-aided design (CAD) modelby causing the processor to: store the CAD model in the memory medium,wherein the CAD model comprises a plurality of input surfaces comprisingfirst and second input surfaces that cross in space along at least oneintersection curve shared by the first and second input surfaces,wherein the first input surface is described in a first parameter spacedomain, and wherein the second input surface is described in a secondparameter space domain, store at least one model space trim curve in thememory medium, wherein the at least one model space trim curve are oneor more curves that represent the at least one intersection curve sharedby the first and second input surfaces; construct first and secondoutput surfaces, wherein the first and second output surfaces areconstructed based on the first and second input surfaces and the atleast one model space trim curve, wherein the first and second outputsurfaces exhibit increased inter-surface continuity relative to thefirst and second input surfaces, and wherein the first and second outputsurface are described in a third parameter space domain; and replace, inthe CAD model, the first and second input surfaces with the first andsecond output surfaces to produce a modified CAD model, wherein themodified CAD model exhibits increased inter-surface continuity betweenthe first and second output surfaces relative to the first and secondinput surfaces; and store the modified CAD model in the memory medium.36. The non-transitory computer-readable memory medium of claim 35,wherein the program instructions are further executable by the processorto cause the processor to: identify a series of points along the atleast one model space trim curve; and construct a series of isocurvesbased at least in part on the identified series of points, wherein themodified CAD model is constructed further based at least in part on theseries of isocurves.